do you want to allow an empty var or are you complaining that by changing to empty var it keeps the old value?
see also #215

To allow an empty var.

This came up where a global variable was set

x='y'

but then it seems not to be possible to subsequently effectively unset the variable in hostgroup or host.
Being able to set

x=''

would do the trick I believe. So this probably is a duplicate of #215.

Steve and Jan,