Specialize operator<<(packer, T) also for object::with_zone.

It already existed for T = object, but if trying to pass an
object::with_zone it would match the generic operator<<() instead.
That operator calls o.msgpack_pack(), which doesn't exist and
causes a compile error.

(Re-)Using the overload for object is a better way to handle this.

src/msgpack/object.hpp

@@ -412,6 +412,12 @@ packer<Stream>& operator<< (packer<Stream>& o, const object& v)
 	}
 }
 
+template <typename Stream>
+packer<Stream>& operator<< (packer<Stream>& o, const object::with_zone& v)
+{
+	return o << static_cast<object>(v);
+}
+
 
 }  // namespace msgpack