message: Add type-check to attach_stream_source
diff: #include "caf/is_actor_handle.hpp"
  #include "caf/make_source_result.hpp"
  #include "caf/policy/arg.hpp"
  +#include "caf/response_type.hpp"
  #include "caf/stream_source.hpp"
  namespace caf {
  @@ -119,7 +120,11 @@ detail::enable_if_t<is_actor_handle<ActorHandle>::value,
  attach_stream_source(scheduled_actor* self, const ActorHandle& dest,
  std::tuple<Ts...> xs, Init init, Pull pull, Done done,
  Finalize fin = {}, policy::arg<DownstreamManager> = {}) {
  - // TODO: type checking of dest
  + using namespace detail;
  + using token = type_list<stream<typename DownstreamManager::output_type>,
  + strip_and_convert_t<Ts>...>;
  + static_assert(response_type_unbox<signatures_of_t<ActorHandle>, token>::valid,
  + "receiver does not accept the stream handshake");
  using driver = detail::stream_source_driver_impl<DownstreamManager, Pull,
  Done, Finalize>;
  auto mgr = detail::make_stream_source<driver>(self, std::move(init),
  