object: Don't let a method shadow a property

This changed due to a change in property access code in SpiderMonkey 45.
Previously, the getProperty hook was apparently called for every property
access. Now, it's never called if the resolve hook defines a lazy
property first.

This is unfortunate; we may still change this behaviour (see
https://bugzilla.gnome.org/show_bug.cgi?id=690450#c30), but if we do
change it then we want it to be intentional.

https://bugzilla.gnome.org/show_bug.cgi?id=785091