[BUGFIX beta] Prevent mandatory-setter when setter is already present.

Closes #9899.
Fixes #9897.
I don't think so.  In the following cases the `isWritable` and `hasValue` both default to true:

* we are working on a `Object.create(null)` object
* we are working with a property that is inherited from a parent in the prototype chain

In those scenarios we should still install mandatory-setter.