return 304 from TransformingTransport
return 304 from TransformingTransport

If the caching headers in the request are valid, return a 304 response
instead of doing the transformation.

Ref #92

wojtekzw/imageproxy,willnorris/imageproxy,betabrand/imageproxy,betabrand/imageproxy,wojtekzw/imageproxy,willnorris/imageproxy