[WIP] Upgrade to rails master/4.2

I was going to send this when I have the complete set of patches required, but I noticed @SamSaffron has started working on it too, so we should probably share notes :smile:
yeah, it was a quick hack: https://github.com/chancancode/discourse/commit/ec0240dda1d182a31d4d0c0290d8acfa1309c9cb

This broke because the method now returns a proxy to make `deliver_later` work. I believe it's better to just rewrite this without relying on the internal `NullMail` anyway which is why I just hacked it (I didn't dig deep enough to know what this is supposed to be testing to rewrite that)