code: func (r *Request) StringProperty(k string) string {
  	if r.Properties == nil {
  		return ""
  	}
  	prop := r.Properties.Fields[k]
  	if prop == nil {
  		return ""
  	}
  	return prop.GetStringValue()
  }
docs: // StringProperty returns a value of string property or "" if no such property
  // or it has a different type.
  //
  // This is useful in tests for asserts.