Make middleware comply with the expected shape

[mrcasals]

This PR changes the Mux.Middleware.SimplifyResponse to comply with the expected shape of 2-element tuples. This PR closes #29.

Base idea is taken from elixir-tesla/tesla#483 (comment)

Please, feel free to change the name of the fields in the middleware!

If this gets merged, this might require a major version bump because it changes the expected API of the library.

[mrcasals]

For some reason, PRs #31 and this one are not running the tests on CI, could you check it out please? @mmcc @dylanjha maybe?

Tests pass locally, but it's better if we could run them on the cI!

[mrcasals]

I've rebased this PR so that we can use GitHub Actions here and ensure the CI is green. See #33 for more info on that.

[mupkoo]

I created a PR with a different solution to the problem, which is using the same API as the current version. Feel free to take a look #34

[mrcasals]

@mupkoo you approach looks good to me too, I'll leave it to the maintainers to decide what they prefer to merge!

[mrcasals]

Actually, I'd probably go with @mupkoo's approach. The idea to keep the middleware compatible with the rest of Tesla's middlewares (as in my PR) might not be actually useful. Wrapping the Tesla public API into a Facade pattern and interacting with it seems to me like a more logical approach, since it gives a better encapsulation and removes the need of using the middleware (which took me a while to figure out because middlewares are not obvious pieces of software).

So I'm closing this PR so that efforts can be focused on @mupkoo's #34.

Thanks!