fix - rotating parent should rotate child

Previous behavior: Rotating parent caused child rotation to be undefined
Now: Rotating parent rotates the child as expected.

Added unit test that failed before, but passes now.

See issue #378.

Unlike the proposal in #378, this change does not have a side-effect
that messes up collision polygon rotation. (I checked that collision
polygon rotation was correct both before and after this change.)
Merge pull request #450 from sbyrnes321/rotation_sign_error_take2

fix - rotating parent should rotate child