fix(tagsInput): pressing tab and selecting an autocomplete result throws digest error

If you manage to blur input while autocomplete is open and than click
an auto-complete result you get a digest in progress error.

Reproduce plunkr: http://plnkr.co/edit/4Gn77U?p=preview
Required environment:
ngTagsInput with the setting add-on-blur="false" and another input.

Reproduce steps:
Write something to autocomplete and make it visible.
Press tab button. The second inputs should get focus.
Now click to an autocomplete result BY mouse. This triggers a focus
event to input.
Focus event to input calls $scope.apply() if not already focused.
However in this flow it is not focuces. So, it tries to apply. However,
It was already started on click.
Check your console to see the error.

This fixed by checking digest status before calling $apply