[5.3] Properly handle filling JSON attributes on Models

[themsaid]

In reference to #16213

When we do:

$user = User::find(1)->update( ['meta->area' => 'new value'] );

The user object after the update will look like

    {
        meta : {area : 'old value'},
        meta->area : "new value"
    }

This PR fills the value of meta correctly and unsets the meta->area attribute, this will present correct output for the model after saving.

[lukepolo]

Awesome thanks for tackling this.