rfc822: Allow test infrastructure to handle headers with minor errors

rfc822: Allow test infrastructure to handle headers with minor errors

Currently the test infrastructure for constructing example messages then
parsing them assumes that constructed headers never have errors of any
kind.  That's true so far, but it limits the versatility of this test
apparatus.  This patch extends the infrastructure to allow minor errors
(that is things other than a missing colon) to be handled.  The existing
test data is also extended to include cases which use this.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>