Note that the parser does attempt to accept obsolete syntax (registering defects for it), so if there is a bug in the implementation of the obsolete syntax handling it should be fixed.  And yes, there have been other bugs with whitespace handling in the parser, unfortunately.

Examples would be most helpful, even if you don't write unit tests.  Most of the tests, by the way, are in test__header_value_parser (search for message_id).  There aren't very many, so more would be good.