Fix cache key not depending on chosen_algorithm

[xmcp]

This PR fixes #53

[kurtmckee]

It looks like the README needs to be updated as well -- it appears to be documenting broken caching behavior, but please double-check me on that.

[alexprengere]

The current README documents this:

def get_cache_key(request):
    return request.url

Do you mean that this is broken as request.url is not enough to build a key?
I agree if that is what you mean, but building such a key for general purpose is not trivial, and really depends on the use case.

[kurtmckee]

I just looked at the updated code; looks like whatever is returned by the get_cache_key() call is then augmented internally by adding the compression algorithm.

If my understanding is correct, then there's no need to modify the README. 👍