[Bug 1673307] Re: msg_header and/or msg_footer can be added as a separate MIME part even if only whitespace.

** Description changed:

If a content filtered message is other than single part text/plain, msg_header and msg_footer are added as separate MIME parts if they are non-empty even if they consist only of white space. msg_header and msg_footer consisting of only white space should be treated as empty and not added. + + Similar considerations apply to digest_header and digest_footer in both + MIME and, for footer at least, plain digests