HORNETQ-1219 Fix inconsistent 'null' property behavior.

Spec says setObjectProperty(...) should be the same as 'typed' alternatives.
Notice that our implementation of the typed variants do not ignore 'null'
values.
Merge pull request #1104 from FranciscoBorges/2.3.x

HORNETQ-1219 Fix inconsistent 'null' property behavior.