message: Fix signature check for set_default_handler
diff: @@ -309,7 +309,7 @@ public:
  /// Sets a custom handler for unexpected messages.
  template <class F>
  typename std::enable_if<std::is_convertible<
  - F, std::function<result<message>(message&)>>::value>::type
  + F, std::function<skippable_result(message&)>>::value>::type
  set_default_handler(F fun) {
  default_handler_ = [=](scheduled_actor*, message& xs) { return fun(xs); };
  }
  