message: Add return statements to append_tuple
diff: @@ -80,7 +80,7 @@ public:
  message_builder& append_tuple(std::integral_constant<size_t, N>,
  std::integral_constant<size_t, N>,
  std::tuple<Ts...>&) {
  - // end of recursion
  + return *this;
  }
  template <size_t I, size_t N, class... Ts>
  @@ -88,12 +88,12 @@ public:
  std::integral_constant<size_t, N> e,
  std::tuple<Ts...>& xs) {
  append(std::move(std::get<I>(xs)));
  - append_tuple(std::integral_constant<size_t, I + 1>{}, e, xs);
  + return append_tuple(std::integral_constant<size_t, I + 1>{}, e, xs);
  }
  template <class... Ts>
  message_builder& append_tuple(std::tuple<Ts...> xs) {
  - append_tuple(std::integral_constant<size_t, 0>{},
  + return append_tuple(std::integral_constant<size_t, 0>{},
  std::integral_constant<size_t, sizeof...(Ts)>{}, xs);
  }
  