negotiator.go
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
}
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
}
// NewJsonXmlContentNegotiator creates a basic ContentNegotiator and attaches
// a JSON and an XML encoder to it.