Don't try to decode the old NSData or NSDate values when the property has been set to nil

The same conditions are used in getValueOfProperty:
Merge pull request #73 from martijnwalraven/master

Don't try to decode the old NSData or NSDate values when the property has been set to nil
Plug mutableCopy leak.
Merge pull request #75 from monowerker/leakplug

Plug mutableCopy leak.
Merge remote-tracking branch 'origin/master' at ae4dd5a into develop. ‘origin/master’ incorporates upstream changes from couchbaselabs/CouchCocoa/master at that commit.