negotiator.go
func (cn *ContentNegotiator) AddEncoder(mimeType string, enc Encoder) {
	if cn.encoderMap == nil {
		cn.encoderMap = make(map[string]Encoder)
	}
	cn.encoderMap[mimeType] = enc
}
func (cn *ContentNegotiator) AddEncoder(mimeType string, enc Encoder) {
	if cn.encoderMap == nil {
		cn.encoderMap = make(map[string]Encoder)
	}
	cn.encoderMap[mimeType] = enc
}
// AddEncoder registers a mimetype and its encoder to be used if a client
// requests that mimetype