message: Add missing inspect overload
diff: @@ -147,6 +147,14 @@ public:
  return value_;
  }
  + // -- serialization ----------------------------------------------------------
  +
  + template <class Inspector>
  + friend typename Inspector::result_type inspect(Inspector& f,
  + outbound_stream_slot& x) {
  + return f(x.value_);
  + }
  +
  private:
  stream_slot value_;
  };
  