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