Fix nested form body parsing happening for all content types

[cllns]

I have a JSON API Hanami app and when benchmarking it, I had some generated code that POSTed with "%" in a field, as in "Improves speeds by 50%". This surprisingly raised a Rack::QueryParser::InvalidParameterError.

This is because we run Rack::Utils.parse_nested_query on the request body for every request, regardless of Content-Type. This causes JSON bodies to leak into router.params (as evidenced by the JSON regression test failure added here: {from: "ui", "{\"foo\":\"bar\"}": nil}), while bodies containing a literal % (#237) or multipart binary bytes (#296, other regression test) raise Rack::QueryParser::InvalidParameterError because it's being parsed as form input (nested query) and the % is interpreted as malformed URL encoding.

#237 was closed with #240 as the workaround, but that was at a different time in the frameworks history: when we required JSON parsing to be done via a body parser in this repo. Now we're more flexible and parse it later on, so the problem gets triggered upstream. This fixes it at its proper place, only using ::Rack::Utils.parse_nested_query for forms.

Closes #296