email: Decode headers when checking .as_string() flatten fallback

A naive dict-comparison fails if any header fields are encoded
following RFC 2047.  For example,

  '=?iso-8859-1?q?this=20is=20some=20text?='

will not compare equal to an email.header.Header instance.  By
decoding everything to Unicode strings before comparing the header
fields, we can see if the underlying data matches.

Signed-off-by: W. Trevor King <wking@tremily.us>