message: Fix tuple_field::GetVariable return type
diff: @@ -19,12 +19,12 @@ struct tuple_field
  static const Metadata metadata;
  static const uint16_t id = Id;
  - static const T& GetVariable(const struct_type& obj)
  + static const value_type& GetVariable(const struct_type& obj)
  {
  return std::get<id>(obj);
  }
  - static T& GetVariable(struct_type& obj)
  + static value_type& GetVariable(struct_type& obj)
  {
  return std::get<id>(obj);
  }
  