Fix trailing args bug in erc-d-i--parse-message

Fix trailing args bug in erc-d-i--parse-message

* test/lisp/erc/resources/erc-d/erc-d-i.el: Require `subr-x'.
(erc-d-i--parse-message): Populate `contents' slot when lone trailing
arg lacks a preceding colon.
* test/lisp/erc/resources/erc-d/erc-d-tests.el (erc-d-i--parse-message):
Fix expected result.
(erc-d-i--parse-message/privmsg): New test.

(cherry picked from commit a5b2de8b54c3a2d7dcb84f7697e18b2257959dfc)