fix(tagsInput): Only updating model through ng-model

Not setting tags explicitly as that circumvents ng-model and can create invalid models on parent, especially when model hasn't been loaded yet (```scope.tags === undefined```). Removed test for testing just that and defaulted .tags to be ```[]``` in tests for them to work the way they were written. Also defaulted to updating BOTH through ```ngModel``` and direct 2-way angular binding on tags, since otherwise a lot of tests would have broken.

Can break code that relies on their model being initialized indirectly by tagsInput. Closes #320