Moves the resetting of _currentTransform.target inside `if (activeGroup)`-block

This solves a problem that occurred if you were transforming (moving, scaling, rotating) a single object
when toJSON()/toObject() was run.

To reproduce the bug, see this fiddle: http://jsfiddle.net/vdb76/1/

Related commit: eb75f4b491c97577d419ab0ee9ca382b37434d29

Related issues:
#1159 (resolved by the commit above)
#896 (possibly fixed)