#5871: protect against header injection attacks.

This makes Header.encode throw a HeaderParseError if it winds up
formatting a header such that a continuation line has no leading
whitespace and looks like a header.  Since Header accepts values
containing newlines and preserves them (and this is by design), without
this fix any program that took user input (say, a subject in a web form)
and passed it to the email package as a header was vulnerable to header
injection attacks.  (As far as we know this has never been exploited.)

Thanks to Jakub Wilk for reporting this vulnerability.