Should be smarter about headers

Issue Summary

Mass Contact creates the entire email, including headers, which includes Content-Type and Bcc. When htmlmail_mail_alter gets called, all the work that module did gets trashed. The entire header array is totally replaced with a single-element array with the key "Content-Type".

Could the functionality of htmlmail be extended s.t. if the message header already contains a Content-Type that indicates HTML, don't replace it?

Also, could htmlmail preserve all those other important header fields that we had before its mail_alter got called?

How about this logic:
When htmlmail_mail_alter gets called, see if $message is a content type of "multipart*". If it *is*, then see if the body contains a content-type of text/html somewhere in the body. If it *does*, then skip the construction of the header.

On the other hand, if $message does have a header, but that header doesn't already specify a content-type, just *add* the content-type value, don't totally replace everything in the header.

Is this an inherently unsafe approach?

Thanks