Support Rails' `parsed_body` in Integration tests

[tagliala]

Ref: https://api.rubyonrails.org/classes/ActionDispatch/TestResponse.html

Close #140

[remear]

Thanks for this @tagliala.

I wasn't able to get this fully working in my testing until I added a param_encoder to the register_encoder call.

[raldred]

Any chance of a release @tagliala ?

[tagliala]

Hi, I'm not an owner / maintainer. I'm referencing to master branch in the Gemfile from a while