Re-rename of mailbox fails.

[nning]

If a mailbox "foo" is renamed to "bar" and then to "foo" again, an exception is thrown (see test/unit/mailbox_test.rb:57):

SQLite3::ConstraintException: UNIQUE constraint failed: relocations.mailbox_id:
INSERT INTO "relocations" ("created_at", "mailbox_id", "old_domain",
"old_username", "updated_at") VALUES (?, ?, ?, ?, ?)