parser: deal with headers entirely failing to parse

parser: deal with headers entirely failing to parse

It turns out that the attempts in clean_header() to convert
headers to strings are not guaranteed to work: you can end up with,
for example, a base64 decoding error which makes it impossible
to determine any header content.

In this case, sanitise_header() should return None, and thus
clean_header() should return None. We then need to plumb that
through.

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