* bzrlib/merge_directive.py: (MergeDirective.to_email): Use EmailMessage instead of email.Message.
* bzrlib/tests/test_merge_directive.py, bzrlib/tests/blackbox/test_merge_directive.py: (__main__): adjust EMAIL1 and EMAIL2 strings to how EmailMessage formats itself.
* bzrlib/smtp_connection.py: (SMTPConnection.get_message_addresses): do not use methods present in email.Message but not in EmailMessage (get_all). Use get() instead of __getitem__ to make explicit that None is returned if the header does not exist.
* bzrlib/tests/test_smtp_connection.py: (TestSMTPConnection.test_get_message_addresses, TestSMTPConnection.test_destination_address_required): test the functions against EmailMessage in addition to email.Message.