mailer.rb: Fix message-id generation for outgoing emails.

Without this fix, if one configures Redmine to use something like «Redmine <redmine@example.com>» as the outgoing address
(which works nicely, exept for the message-id), the message-id is set to invalid string like <redmine.journal-numbers@example.com>>
becase of the invalid hostname extraction.
This results in Google ignoring the invalid message-id field and replacing it with an autogenerated one.
The message then fails DKIM check and goes into the spam folder.

This commit fixes invalid message-id generation, and everything works fine for me now.