a11y: Remove an overzealous optimisation

The result of calling update_property needs to be that the property is marked as set afterward, even if the value we pass happens to match the default value.

After this change, scrollbars have value-now show up as zero in the accessiblity page of the inspector, even when that matches the lower bound.

This is relevant e.g. in the a11y overlay which checks for required attributes and will complain if they go missing due to this optimization.

Fixes: #5886 (closed)