Content-Encoding with multiple encodings not supported

[jbramleycl]

It is possible for a server to respond with a Content-Encoding header like:
Content-Encoding: deflate, deflate

From RFC7231:

If one or more encodings have been applied to a representation, the
sender that applied the encodings MUST generate a Content-Encoding
header field that lists the content codings in the order in which
they were applied.

This can be tested with:

def test_multiple_content_encodings():
    data = zlib.compress(zlib.compress(b'foo'))
    fp = BytesIO(data)
    r = HTTPResponse(fp, headers={'Content-Encoding': 'deflate, deflate'})
    assert r.data == b'foo'

Would it make sense to do something like:

if len(self.headers.get('content-encoding').split(',') > 1:
    self._decoder = MultiDecoder(self.headers.get('content-encoding'))

where MultiDecoder just iterates through the list applying standard decoders?

[sethmlarson]

TIL that Content-Encoding allows for nested encodings. Do you have an example site that does this? We haven't hit this problem until now.

I'm open to a PR that implements this behavior.