code: func (p Params) Add(k, v string) {
  	if param, ok := p[k]; ok {
  		switch param.(type) {
  		case string:
  			param = []string{param.(string), v}
  		case []string:
  			param = append(param.([]string), v)
  		}
  		p[k] = param
  	} else {
  		p[k] = v
  	}
  }
docs: // Add param to Params