Fix breaking of rfc2047 headers with long words

Fix breaking of rfc2047 headers with long words

* lisp/mail/rfc2047.el (rfc2047-fold-region): Don't break lines
right after the Header: field, but wait until the next
whitespace.  (This only makes a difference for words that are very
long (i.e., longer than, say, 60 characters, depending on the
header name length.)