Plack::Request: die on superfluous arguments

I had some code that accidentally did
$plack_request->content_type($type) instead of
$plack_response->content_type($type), this just silently failed
instead of issuing a warning about the unknown argument.

I think Plack should die instead when you supply it with parameters
that it can't possibly do anything with. This is a partial hack to get
that going, this is just a proof of concept patch but a good start. In
order to fully execute on this we'd need to go through the whole plack
source and start dying on any unknown arguments.

This patch is best viewed with the --word-diff argument to git-show.