This is what I tried (but now with your example) but keep getting the same error.

curl -s -H "Accept:application/json" -k -u admin:changeme -X POST {"name":"foo","host":"foo.example.com"} https://foreman.localdomain/api/v2/auth_source_ldaps
{
  "error": {"id":null,"errors":{"name":["can't be blank"],"host":["can't be blank"]},"full_messages":["Name can't be blank","Server can't be blank"]}
}
[root@foreman foreman]# curl -s -H "Accept:application/json" -k -u admin:changeme -X POST {"name":"foo","host":"foo.example.com"} https://foreman.localdomain/api/v1/auth_source_ldaps
{"auth_source_ldap":{"id":null,"errors":{"name":["can't be blank"],"host":["can't be blank"]},"full_messages":["Name can't be blank","Server can't be blank"]}}

Started POST "/api/v2/auth_source_ldaps" for 192.168.88.135 at 2014-03-03 02:13:12 -0800
Processing by Api::V2::AuthSourceLdapsController#create as JSON
  Parameters: {"apiv"=>"v2"}
  ^[[1m^[[36mUser Load (0.7ms)^[[0m  ^[[1mSELECT "users".* FROM "users" WHERE "users"."login" = 'admin' LIMIT 1^[[0m
  ^[[1m^[[35mAuthSource Load (0.5ms)^[[0m  SELECT "auth_sources".* FROM "auth_sources" WHERE "auth_sources"."id" = 1 LIMIT 1
  ^[[1m^[[36mCACHE (0.0ms)^[[0m  ^[[1mSELECT "users".* FROM "users" WHERE "users"."login" = 'admin' LIMIT 1^[[0m
Authenticated user Admin User against INTERNAL authentication source
  ^[[1m^[[35mUser Load (0.5ms)^[[0m  SELECT "users".* FROM "users" WHERE "users"."login" = 'admin' ORDER BY firstname LIMIT 1
Setting current user thread-local variable to admin
  ^[[1m^[[36m (0.2ms)^[[0m  ^[[1mBEGIN^[[0m
  ^[[1m^[[35m (0.5ms)^[[0m  UPDATE "users" SET "last_login_on" = '2014-03-03 10:13:12.084128', "updated_at" = '2014-03-03 10:13:12.085458' WHERE "users"."id" = 1
  ^[[1m^[[36m (1.1ms)^[[0m  ^[[1mCOMMIT^[[0m
  ^[[1m^[[35mRole Load (0.5ms)^[[0m  SELECT "roles".* FROM "roles" WHERE "roles"."name" = 'Anonymous' LIMIT 1
  ^[[1m^[[36m (0.3ms)^[[0m  ^[[1mSELECT "roles".id FROM "roles" INNER JOIN "user_roles" ON "roles"."id" = "user_roles"."role_id" WHERE "user_roles"."user_id" = 1^[[0m
  ^[[1m^[[35mCACHE (0.0ms)^[[0m  SELECT "roles".* FROM "roles" WHERE "roles"."name" = 'Anonymous' LIMIT 1
  ^[[1m^[[36mRole Exists (0.3ms)^[[0m  ^[[1mSELECT 1 AS one FROM "roles" INNER JOIN "user_roles" ON "roles"."id" = "user_roles"."role_id" WHERE "user_roles"."user_id" = 1 AND "roles"."id" = 8 LIMIT 1^[[0m
Setting current user thread-local variable to admin
Setting current user thread-local variable to nil
  ^[[1m^[[35mUser Load (0.3ms)^[[0m  SELECT "users".* FROM "users" WHERE "users"."login" = 'admin' LIMIT 1
Authorized user admin(Admin User)
Setting current user thread-local variable to admin
  ^[[1m^[[36m (0.1ms)^[[0m  ^[[1mBEGIN^[[0m
  ^[[1m^[[35mAuthSource Exists (0.3ms)^[[0m  SELECT 1 AS one FROM "auth_sources" WHERE "auth_sources"."name" = '' LIMIT 1
  ^[[1m^[[36m (0.1ms)^[[0m  ^[[1mROLLBACK^[[0m
Unprocessable entity AuthSourceLdap (id: new):
  Name can't be blank
  Server can't be blank

  Rendered api/v2/errors/unprocessable_entity.json.rabl within api/v2/layouts/error_layout (0.9ms)
Body: {
  "error": {"id":null,"errors":{"name":["can't be blank"],"host":["can't be blank"]},"full_messages":["Name can't be blank","Server can't be blank"]}
}

Completed 422 Unprocessable Entity in 22ms (Views: 1.4ms | ActiveRecord: 5.5ms)

Started POST "/api/v1/auth_source_ldaps" for 192.168.88.135 at 2014-03-03 02:13:47 -0800
Processing by Api::V1::AuthSourceLdapsController#create as JSON
  Parameters: {"apiv"=>"v1"}
  ^[[1m^[[35mUser Load (0.5ms)^[[0m  SELECT "users".* FROM "users" WHERE "users"."login" = 'admin' LIMIT 1
  ^[[1m^[[36mAuthSource Load (0.3ms)^[[0m  ^[[1mSELECT "auth_sources".* FROM "auth_sources" WHERE "auth_sources"."id" = 1 LIMIT 1^[[0m
  ^[[1m^[[35mCACHE (0.0ms)^[[0m  SELECT "users".* FROM "users" WHERE "users"."login" = 'admin' LIMIT 1
Authenticated user Admin User against INTERNAL authentication source
  ^[[1m^[[36mUser Load (0.4ms)^[[0m  ^[[1mSELECT "users".* FROM "users" WHERE "users"."login" = 'admin' ORDER BY firstname LIMIT 1^[[0m
Setting current user thread-local variable to admin
  ^[[1m^[[35m (0.2ms)^[[0m  BEGIN
  ^[[1m^[[36m (0.3ms)^[[0m  ^[[1mUPDATE "users" SET "last_login_on" = '2014-03-03 10:13:47.754480', "updated_at" = '2014-03-03 10:13:47.755214' WHERE "users"."id" = 1^[[0m
  ^[[1m^[[35m (0.7ms)^[[0m  COMMIT
  ^[[1m^[[36mRole Load (0.3ms)^[[0m  ^[[1mSELECT "roles".* FROM "roles" WHERE "roles"."name" = 'Anonymous' LIMIT 1^[[0m
  ^[[1m^[[35m (0.3ms)^[[0m  SELECT "roles".id FROM "roles" INNER JOIN "user_roles" ON "roles"."id" = "user_roles"."role_id" WHERE "user_roles"."user_id" = 1
  ^[[1m^[[36mCACHE (0.0ms)^[[0m  ^[[1mSELECT "roles".* FROM "roles" WHERE "roles"."name" = 'Anonymous' LIMIT 1^[[0m
  ^[[1m^[[35mRole Exists (0.3ms)^[[0m  SELECT 1 AS one FROM "roles" INNER JOIN "user_roles" ON "roles"."id" = "user_roles"."role_id" WHERE "user_roles"."user_id" = 1 AND "roles"."id" = 8 LIMIT 1
Setting current user thread-local variable to admin
Setting current user thread-local variable to nil
  ^[[1m^[[36mUser Load (0.3ms)^[[0m  ^[[1mSELECT "users".* FROM "users" WHERE "users"."login" = 'admin' LIMIT 1^[[0m
Authorized user admin(Admin User)
Setting current user thread-local variable to admin
  ^[[1m^[[35m (0.1ms)^[[0m  BEGIN
  ^[[1m^[[36mAuthSource Exists (0.3ms)^[[0m  ^[[1mSELECT 1 AS one FROM "auth_sources" WHERE "auth_sources"."name" = '' LIMIT 1^[[0m
  ^[[1m^[[35m (0.1ms)^[[0m  ROLLBACK
Unprocessable entity AuthSourceLdap (id: new):
  Name can't be blank
  Server can't be blank

  Rendered api/v1/errors/unprocessable_entity.json.rabl (1.0ms)
Body: {"auth_source_ldap":{"id":null,"errors":{"name":["can't be blank"],"host":["can't be blank"]},"full_messages":["Name can't be blank","Server can't be blank"]}}
Completed 422 Unprocessable Entity in 16ms (Views: 1.6ms | ActiveRecord: 4.1ms)

The logging from earlier post was created when I switched from command line to testing with google chrome postman.

I might have forgotten to pass the application/json param/header there but didn't notice because I got same error result as when using command line.

This error also occurs when using -d between POST and json. (I see I forgot them in the tests above)

Ok, yes that works :)

Totally forgot that one :(

Please close this one :)