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.