object: Fix write-only properties

Since the property refactor, write-only properties have not been working. The problem was that a getter and setter function were not defined for them, because is_gobject_property_name() did not consider them to be properties. Now, the setter function works as normal while the getter function just pretends a write-only property has the value of undefined.

The test is marked pending until a test is added to the gobject-introspection test suite.