parser: subject_check needs to call clean_header

parser: subject_check needs to call clean_header

When testing with headers with unencoded characters on Py3,
subject_check would fail. It was getting an Header class, rather
than a str/bytes, and so the regex was failing.

Run clean_header to decode everything fully first.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Stephen Finucane <stephen@that.guru>