parser: don't assume headers are strings

parser: don't assume headers are strings

In python3, mail.get() can return either a string, or an
email.header.Header type.

clean_header() is designed to clean headers into strings,
so make sure we use that everywhere.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Stephen Finucane <stephen@that.guru>