Improve rack.after_reply

[casperisfine]

Things to do:

• Don't trust that callbacks don't raise. If one raise, swallow the error and continue calling the next one.
• This capability was added to the rack spec under another name: Can rack.after_reply be added to the spec? rack/rack#1777 / Add rack.response_finished to Rack::Lint. rack/rack#1952, might be worth aliasing the env key.