old_contents: import msgpack
  
  class MsgPackRenderer(object):
      def __init__(self, info):
          pass
      def __call__(self, value, system):
          request = system.get('request')
          if request is not None:
              response = request.response
              ct = response.content_type
              if ct == response.default_content_type:
                  response.content_type = 'application/x-msgpack'
          return msgpack.packb(value)
  
  import ujson
  class UjsonRenderer(object):
      def __init__(self, info):
          pass
      def __call__(self, value, system):
          request = system.get('request')
          if request is not None:
              response = request.response
              ct = response.content_type
              if ct == response.default_content_type:
                  response.content_type = 'application/x-msgpack'
          return ujson.dumps(value)
  
  
new_contents: import msgpack
  
  class MsgPackRenderer(object):
      def __init__(self, info):
          pass
      def __call__(self, value, system):
          request = system.get('request')
          if request is not None:
              response = request.response
              ct = response.content_type
              if ct == response.default_content_type:
                  response.content_type = 'application/x-msgpack'
          return msgpack.packb(value)
  
  import ujson
  class UjsonRenderer(object):
      def __init__(self, info):
          pass
      def __call__(self, value, system):
          request = system.get('request')
          if request is not None:
              response = request.response
              ct = response.content_type
              if ct == response.default_content_type:
                  response.content_type = 'application/x-json'
          return ujson.dumps(value)
  
  
subject: correct content type for ujson renderer