_split_header(): The code here was terminally broken because it didn't

_split_header(): The code here was terminally broken because it didn't
know anything about RFC 2047 encoded headers.  Fortunately we have a
perfectly good header splitter in Header.encode().  So we just call
that to give us a properly formatted and split header.
Header.encode() didn't know about "highest-level syntactic breaks" but
that's been fixed now too.