Test this, both in strict and relaxed mode.

Also add more tests.

Also add better error handling, and tests for strict and relaxed
behaviour.

That means switching from nose to pytest.

pytest-localserver is a pytest plugin providing an http server
stub. The plugin doesn't work with Python3 however, and has to
be blacklisted in tox.

I'm not sure how this relates to other mime uses, like e-mail.

2to3 is enabled by switching to Distribute from distutils.

LEPL now works on character strings; I'd prefer byte strings,
but this seems to work.

The bugs happened when Content-Disposition isn't defined.