When replying to a post, propagate its spoiler warning.

When replying to a post, propagate its spoiler warning.

I forgot this yesterday in discussions of a maths problem, and posted
an answer in cleartext. Happily, it wasn't a very hard problem. But it
would have been better if the default had been to repeat the spoiler
text from the parent post; I could always change it manually, but if I
don't want to think very hard, this would more likely be right.