Feature/unset attributes when set to null

https://versal.atlassian.net/browse/GAD-23
https://github.com/Versal/versal-gadget-api/issues/27

unset `temp` attribute by doing `setAttributes({temp: null})`
Well you never set `testNull` in the first place. You can instead test that the attribute `test` disappears: 
`launcher.children[0].contentWindow.sendGadgetEvent({event: 'setAttributes', data: {test: null}});`