LibJS: Use existing attributes if any are missing in the new descriptor

The specification defines that we should only change attributes that
exist in the incoming descriptor, but since we currently just overwrite
the existing descriptor with the new one, we can just set the missing
attributes to the existing values manually.

Userland/Libraries/LibJS/Runtime/Object.cpp

@@ -679,6 +679,20 @@ bool Object::put_own_property(const StringOrSymbol& property_name, Value value,
         }
     }
 
+    // FIXME: Instead of adding back existing attributes we should just stop overwriting them
+    if (!attributes.has_configurable() && metadata.value().attributes.is_configurable()) {
+        attributes.set_has_configurable();
+        attributes.set_configurable();
+    }
+    if (!attributes.has_enumerable() && metadata.value().attributes.is_enumerable()) {
+        attributes.set_has_enumerable();
+        attributes.set_enumerable();
+    }
+    if (!value.is_accessor() && !attributes.has_writable() && metadata.value().attributes.is_writable()) {
+        attributes.set_has_writable();
+        attributes.set_writable();
+    }
+
     if (mode == PutOwnPropertyMode::DefineProperty && attributes != metadata.value().attributes) {
         if (m_shape->is_unique()) {
             m_shape->reconfigure_property_in_unique_shape(property_name, attributes);