Accessing request params via Rack::Accept::Request

Rack::Accept::Request overrides media_type but changes the return value. Instead of returning the request's content-type -less charset, like its parent, it returns an accept header (as an instance of MediaType). This prevents one from retrieving POST params:

[Rack::Accept::Request, Rack::Request].each do |klass|
  env = {    
    "rack.input"     => StringIO.new("a=b+c"), 
    "CONTENT_TYPE"   => "multipart/form-data",
    "REQUEST_METHOD" => "POST"
  }

  r = klass.new(env)  
  puts "#{klass.name}#POST: #{r.POST}"
  puts "#{klass.name}#media_type: #{r.media_type}"   
  puts "*" * 10
end

Prints:

Rack::Accept::Request#POST: {}
Rack::Accept::Request#media_type: Accept: 
**********
Rack::Request#POST: {"a"=>"b c"}
Rack::Request#media_type: multipart/form-data
**********

Maybe it's better to call the method accept

Start at Rack::Accept::Request#media_type and compare it with Rack::Request#media_type, using the reproduction in the issue to confirm how the override affects POST parsing. Add regression coverage for request parameters and verify that media type and Accept handling produce the intended values.