Fix parameterize missing test/params detection.

Which reveals a bug in test_message, so fix that too.
The "provided content managers" are instances.
Add tests for order of set/get key resolution.

Also renamed the 'Message' test classes to 'Thing' to avoid confusion.
(Originally I was really using a Message object, but that changed when
needed something with a different qualname, and I negligently continued
to use the same name for it.
Documentation for raw_content_manager.

I haven't checked how this renders yet.
raw_content_manager.get_content for text parts.
raw_data_manager.get_content for non-text.
Test for get_content raise on multipart.
raw_data_manager get for message maintype.

This took two handlers, since we are supposed to treat unknown
types as application/octent0-streat per the RFC, and there's no
way that 'partial' should be treated as a message object.

Also dropped the decorators as they aren't very useful, and decided
not to support errors on the non-text handlers.  It can be added
later if needed, but not deleted later if I put it in.

Interestingly, everything except the message handler works fine
with compat32.  Even the message handler does, for that matter,
you just get a compat32 message, which is to be expected.