Reset handle in weak callback.

This doesn't seem to be neccesary, but we had better follow what upstream does.

native_mate/wrappable.cc

@@ -44,6 +44,7 @@ ObjectTemplateBuilder Wrappable::GetObjectTemplateBuilder(
 // static
 MATE_WEAK_CALLBACK(Wrappable::WeakCallback, v8::Object, Wrappable) {
   MATE_WEAK_CALLBACK_INIT(Wrappable);
+  self->wrapper_.Reset();
   delete self;
 }