Revert invalid spec change from 034f132f7c49382bd61bd9bad3ce3c48f498b60d
Fix Message comparison method to ensure there's no side-effects.

Comparing a Message w/o message_id set to another object would set it's
Message-ID to a temporary value.  Ditto for the object compared to.
Merge pull request #603 from commandprompt/fix-message-comparison-take2

Fix message comparison: take 2