Don't submit non-filterable bodies

[JoelQ]

🧰 What's being changed?

When given allow_only or reject parameters, we want to make sure
that the request and response bodies are properly filtered. In order to
do that, we need to be able to parse them. Currently we only support
parsing JSON and form-encoded bodies. If bodies come in as other content
types and a filter is configured, we should not submit the request to
the Readme API because it might contain sensitive data.

If no filter is set, it is safe to just pass-through any bodies to the
Readme API regardless of content type.

Testing this behavior was rather annoying as I had to create new
instances of both app and middleware for each test in order to get
custom response bodies and middleware configurations desired. To make
this nicer I ended up creating a bunch of testing helpers. I also
created two test app classes based on content returned: JsonApp and
TextApp.

The checking we do around content-types has to happen for both the
request and the response so I pulled it out into a mixin that can be
included into both. Also, as part of this I created an HttpResponse
class that decorates Rack::Request and includes the mixin.

🧪 Testing

Create a simple rack app that returns text/plain content and has a reject or allow-only parameter set. When you make a request to it, you should see an error in the logs:

Try various combinations of reject_params, allow_only, text response bodies, and text request bodies.