proxy non-200 responses with original status code
proxy non-200 responses with original status code

Some sites return a 404 image, and most browsers render them properly.
We should pass them along as well.

see also: atmos/camo#75 and atmos/camo#77

betabrand/imageproxy,willnorris/imageproxy,zischwartz/imageproxy,wojtekzw/imageproxy,runemadsen/imageproxy,victortrac/imageproxy,WatchBeam/imageproxy,willnorris/imageproxy,WatchBeam/imageproxy,sevki/imageproxy,wojtekzw/imageproxy,victortrac/imageproxy,betabrand/imageproxy,Toubib/imageproxy,sevki/imageproxy,zischwartz/imageproxy,runemadsen/imageproxy,Toubib/imageproxy