test(datepicker): initialize $sniffer variable

The uninitialized $sniffer variable causes the
changeInputValueTo function to fail.

Closes angular-ui#3630

Author: RobJacobs

src/datepicker/test/datepicker.spec.js

@@ -1178,6 +1178,7 @@ describe('datepicker directive', function () {
     describe('initially', function () {
       beforeEach(inject(function(_$document_, _$sniffer_) {
         $document = _$document_;
+        $sniffer = _$sniffer_;
         $rootScope.isopen = true;
         $rootScope.date = new Date('September 30, 2010 15:30:00');
         var wrapElement = $compile('<div><input ng-model="date" datepicker-popup><div>')($rootScope);