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`