Add support for RFC 2231: "MIME Parameter Value and Encoded Word Extensions"

[rlebeau]

When an email uses RFC 2231, eg:

Content-Transfer-Encoding: base64
Content-Type: application/octet-stream;
 name*0*=utf-8''A10090110721%2Epdf
Content-Disposition: attachment;
 filename*0*=utf-8''A10090110721%2Epdf

this useless encoding is not supported and the attachment name is empty.

[rlebeau]

Probably easiest to handle this by updating ExtractHeaderSubItem() to recognize the extended attribute format defined by the RFC so it is transparent to the caller. It already parses a given header into a string list of named attributes, so finding and putting related extended attributes together for a given attribute name should be fairly easy.