Properly handle defaults in org-set-property

[Janek F]

As discussed in 
https://emacs.stackexchange.com/questions/71774/pass-default-value-to-org-set-property/71777,
 providing a default to org-set-property does not always work.

It calls `org-read-property-value`, which only uses the supplied default value 
if it can compute a list of allowed values for the property. But then, the 
default value has to be part of that `_ALL` list.

Can this be changed so it always uses the given default?


Thanks everyone for the great work!
~ Janek