Validate streaming responses from sent chunks

[ydah]

Streaming response validation runs from a Rack::BodyProxy close callback and currently re-enumerates the original response body. Rack closes that body before invoking the callback, so one-shot bodies validate as empty and close-sensitive bodies raise IOError after the response has already been sent.

Wrap streaming enumeration to collect the chunks yielded to the client, then validate those captured chunks from the close callback. The original response body is still closed before validation.

[geemus]

@ydah broadly looks good. I have some concern about memory bloat though, especially if for any reason the extra copy of the data doesn't get garbage collected promptly. Any thoughts on how we could mitigate that or at least identify if it is a real concern?