code: func NewJsonXmlContentNegotiator(defaultEncoder Encoder, responseWriter http.ResponseWriter, prettyPrint bool) *ContentNegotiator {
  	result := NewContentNegotiator(defaultEncoder, responseWriter)
  	result.AddEncoder(MimeJSON, JsonEncoder{prettyPrint})
  	result.AddEncoder(MimeXML, XmlEncoder{prettyPrint})
  	return result
  }
docs: // NewJsonXmlContentNegotiator creates a basic ContentNegotiator and attaches
  // a JSON and an XML encoder to it.