Pass on Message-ID if set when encrypting/signing.

Previously a specified Message-ID was swallowed by the
encrypting/signing-process which made Mail generate new ones.

Now you can specify your own Message-IDs like in stock Mail.
Merge pull request #18 from duckdalbe/pass_message_id

Pass on Message-ID if set when encrypting/signing.
0.2.1
rescue from keyserver errors