Don't pass charset from headers on to fields.

If you do then all the headers after the Content-Type: header will
inherit any encoding set on that line.

At the moment Mail::Field.new defaults to UTF-8 which is the arguably
the correct value (according to RFC 5335).

We could attempt to keep headers as BINARY if we're not mutating them,
however this would require annoyingly large changes; and the practical
benefit is not clear as the mail gem seems to silently work with invalid
UTF-8 in field values (it uses rfc-2047 encoding with "ASCII-8BIT",
and while I doubt that that is a valid thing to do, it probably doesn't
matter for us).
"Fix" UndefinedConversionErrors
Merge remote-tracking branch 'origin/410-no-header-charset' into nmaster
Merge remote-tracking branch 'origin/undefined_conversion_workaround' into nmaster
Don't sort parsed messages [Fixes #497]
Merge branch '497-dont-reorder-parsed' into nmaster
Only add charset= on text parts

Setting the charset for non-text parts is meaningless.
Merge branch 'no-charset-on-image' into nmaster
[HACK] Import rapportive encoding hacks
[HACK] remove call to add_required_fields
[HACK] Allow writing to decoded/raw_source