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

This solves a problem that occurred if you were transforming (moving, scaling, rotating) a single object
when toJSON()/toObject() was run.
Merge pull request #1260 from Webbgaraget/toJSON-currentTransform-bugfix

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