fix main test and bug (#2860)

fixed the main test file which exposed multiple bugs! :

* GridStack:init(null) didn't set this.opts
* loading exiting layout incorrecly moved items (when just one caused collision)
* update(el, {id: 'newId'}) didn't update the id
* some attr setting optimization

going forward need to get full test coverage to help find issues...
continue #2850