type hinting for Mail_mimePart

Yet another feature for Mail_mimePart. If you read RFC2387 (Multipart/Related messages), you can see the following in section 3.1: 3.1. The Type Parameter The type parameter must be specified and its value is the MIME media type of the "root" body part. It permits a MIME user agent to determine the content-type without reference to the enclosed body part. If the value of the type parameter and the root body part's content-type differ then the User Agent's behavior is undefined. I imagine that the word 'must' is a bit too strong - a lot of clients don't implement that when sending messages. Neither does the Mail_mimePart implementation. However, it would simplify the life of the parser on the receiving end, so it would be nice to add that functionality. The patch attached is a rough idea of how it should look like. Please, give feedback and tell me what you think. If you like the idea, I'll test it a bit and make sure I have a patch that's safe to apply. Thanks a lot! Vlad -- Vlad Krupin Software Engineer echospace.com