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mikel
Message#== - comparison strategy
The == method has to generate dummy message-ids if none exist.
As it stands, if either message does not have an id, then both are set to the same fixed value.
This seems fine for the case where neither message has an id, but is it the best strategy when one already has an id and the other does not?
Similarly for Dates (see #1527)
If both messages have ids, the == method invokes encoded without first creating a copy.
This changes the original instance, which does not seem right.
It would be better to always create a duplicate. The duplicate could then have Date and Message-Id added if not present.
There are currently two tests which check that a provided message-id compares equal with nil. These would need to be changed: https://github.com/mikel/mail/blob/bf02b6772e4df20e165c008f595f4eadb8c595a7/spec/mail/message_spec.rb#L1719 and https://github.com/mikel/mail/blob/bf02b6772e4df20e165c008f595f4eadb8c595a7/spec/mail/message_spec.rb#L1730
Allowing Message-Id to match against nil causes unexpected behaviour for the == operator
See https://github.com/mikel/mail/pull/1540
At present, Message equality (==) is not transitive.