Preserve body on certain params parsing

[vladfaust]

The API could look like this:

json require: true, preserve_body: true do
  # ...
end

def call
  context.request.body.gets_to_end # Actual content
end

It would imply copying the body into separate IO, which would, of course, affect the performance. Moreover, it would be impossible to do request body streaming for obvious reason :thinking:

Thanks @qszhu for the proposal.