Small improvement on PhysicsSpace

I changed a little thing in the PhysicsSpace as talked about in [this Thread](http://hub.jmonkeyengine.org/forum/topic/small-improvement-for-physicsspace/#post-293587).

However I **did not** take care of the native Physics Space as I could not find a add(...) method there...
While I use git all the time, I'm new to pull requests, so tell me if I got something wrong. :)