Ensure body is closed inside the proc. Follow existing monkey patch style.

* Serving the rack body is wrapped in begin/ensure/end so that
body.close is guaranteed to be called per rack SPEC. When body
iteration happens inside a proc, the proc has to have its own
begin/ensure/end block to be able to make the same guarantee.
* Made my monkey patch for WEBrick::HTTPResponse follow same style as
the other monkey patch already present in the file and added a comment.