parser: parse headers containing invalid characters or codings

parser: parse headers containing invalid characters or codings

If there is a non-ascii character in a header, parsing fails,
even on Py27.

This has huge Py2/Py3 complexities. The Py3 email package has tools
to handle this - we just need to use them. Py2, on the other hand,
needs a lot of hand-holding, as explained in the comments.

Additionally, support headers that claim an encoding, but fail to
decode with that encoding.

This is handy for mails with malformed headers containing weird
bytes.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reported-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>