Inherited properties are not updatable

[tgallant]

See the comment in the code here https://github.com/lilt/ServiceAPIExtensions/blob/master/ServiceAPIExtensions/Controllers/ContentAPiController.cs#L910-L912

        private string UpdateFieldOnContent(IContent con, string contentName, string propertyName, object value)
        {
            //Problem: con might only contain very few properties (not inherited)
            if (con.Property.Contains(propertyName))
            {
            ...
           }
         }