search: use raw headers without MIME decoding

search: use raw headers without MIME decoding

This should be less error-prone in case somebody tries to screw with
us and our thread_id mechanism or somehow waste our resources.
Unfortunately Mail::Thread isn't smart enough for this, yet, so we
may need to downgrade to Email::Simple objects as a workaround.

Or simply not worry about the display so much if somebody is
intentionally trying to make it thread badly/incorrectly.