parsemail: Fallback to common charsets when charset is None or x-unknown

parsemail: Fallback to common charsets when charset is None or x-unknown

We recently encountered a case in our glibc patchwork instance on
sourceware, where a patch was dropped because it had x-unknown
charset.

This change adds a fallback on a set of encodings (instead of just
utf-8) when the charset is not mentioned or if it is set as x-unknown.

Minor changes and testcase by Jeremy Kerr <jk@ozlabs.org>

Signed-off-by: Siddhesh Poyarekar <siddhesh@redhat.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>