Merge pull request #51 from mailgun/maxim/develop

Make FallbackHeaders interface match with MimeHeaders
Ignore false Content-Type headers

It is ignored in mesage/part bodies as well as in multipart
preambles and epilogues.
Fix a typo in the RichPartMixin name
Merge pull request #50 from mailgun/maxim/tokenizer

Ignore false Content-Type headers in multipart epilogues