[Reviewer: Mike] Don't strip boundary quotes or repeat content headers

Mike, can you review these changes to not strip the quotes on the boundary parameter, and to not repeat the content-* headers

Tested in sprout UTs
This doesn't seem to be very robust to badly formatted bodies - for example, if there is no terminating quote.

PJSIP has a pj_scan_get_quote function that is designed to get the contents of a quoted string, but unfortunately I can't see any way of using it in a context where we don't have a scanner.  However, looking at the code it is significantly more robust than this.

Looking at looking at parse_hdr_content_type it looks like it does check that the m-parameters are either a token or a properly quoted string, so we probably just need a comment making this clear.