Fix MIME headers from exceeding permitted line length

Automatically break a header of outbound MIME content over
multiple lines if needed by calling foldHeader() on each header.

So far outbound MIME content was always assembled with each header
spanning exactly one line, independent of the actual amount of
characters in each header and even if the original inbound content
already had headers correctly folded. This easily caused headers to
exceed the mandatory maximum line length limit of 998 characters as
mandated by RFC 5322 section 2.1.1. "Line Length Limits", which in
turn caused e.g. emails being bounced by many SMTP servers.