client-only tags relayed in DMs even without message-tags cap

[slingamn]

19:46:36 < jess> bug with c2c message tags in private messages: https://pastebin.com/raw/trtKAV50
19:46:48 < jess> i didn't negotiate message-tags but got them anyway
19:47:32 < jess> only appears to happen in private messags, not channel messages

[slingamn]

This goes back a ways, to 1.1 at least (before that it's hard to even run the integration test suite).