Made them consistent with _register_observable_extension, by:
- moving validation logic there from _custom_*_builder functions
- using a new function for ensuring properties are dict-like
- using the library default spec version instead of None
Fix#371, fix#372, fix#373.
TypeProperty uses a fixed value, so check() was never called. This way
also runs the check at object registration time because the wrapper
creates an instance of TypeProperty and doesn't have to wait for the
object to be instantiated so clean() can be called.
Also fix some tests.
Chris Lenk