I noticed that in a few places you're hacking around support for lower case (http2) headers, and there are a bunch of places where you don't handle them correctly. Is there a reason that you're not lowercasing all headers when storing the headers (and also storing the originals for back compat)? eg there are a bunch of places where you test for $response->httpHeaders['Location'] || $response->httpHeaders['location'], but if you standardise them then you can just test one. You already standardise location, but you may as well do it for everything then deprecate the upper case ones.

I've raised your suggestion internally for further consideration.