Handle non-hash body params.

This is related to #347. When the body of the request is not a hash, do not modify the request's form_input since this breaks the Rack::Request#params method by allowing Rack::Request#POST to return nil.
https://github.com/rack/rack/blob/da7efac44c7c463fb39e82d40c4a23403db383de/lib/rack/request.rb#L227
https://github.com/rack/rack/blob/da7efac44c7c463fb39e82d40c4a23403db383de/lib/rack/request.rb#L201