TypeError fixed and all non-collision tests pass.

Collision-based tests fail with a 'NoneType' object not iterable runtime
error.
Fixed SpatialDictionary unit tests and added more

Fixed a _get_nearby_objects() bug where I was discarding a set element,
converting the set to a list, and returning it in one line. But the
return type of discard is not the set itself (it's None). Added basic
tests for the Entity class and HashableRect.