Don't remove :handler from match-context

[daaku]

I traced back the history to 92a2263ad39c61f6812a870a79fe4fe49576899c but it's unclear why :handler is removed from the match-context (along with other info which unlike this is present in the request). I wanted my bidi.ring/Ring implementation to be able to access this match information, and the only way I can do so is to copy paste make-handler and not drop this key. Does it seem reasonable to keep this key in the match-context?

It's a trivial PR if this is acceptable.