fix(tooltip): add safety to $apply

- Change to use `$rootScope.$digest()` only as necessary for safety

Closes #3943
Closes #4150
Fixes #516

src/tooltip/tooltip.js

@@ -65,7 +65,7 @@ angular.module( 'ui.bootstrap.tooltip', [ 'ui.bootstrap.position', 'ui.bootstrap
    * Returns the actual instance of the $tooltip service.
    * TODO support multiple triggers
    */
-  this.$get = [ '$window', '$compile', '$timeout', '$document', '$position', '$interpolate', function ( $window, $compile, $timeout, $document, $position, $interpolate ) {
+  this.$get = [ '$window', '$compile', '$timeout', '$document', '$position', '$interpolate', '$rootScope', function ( $window, $compile, $timeout, $document, $position, $interpolate, $rootScope ) {
     return function $tooltip ( type, prefix, defaultTriggerShow, options ) {
       options = angular.extend( {}, defaultOptions, globalOptions, options );
 
@@ -179,9 +179,10 @@ angular.module( 'ui.bootstrap.tooltip', [ 'ui.bootstrap.position', 'ui.bootstrap
             }
 
             function hideTooltipBind () {
-              scope.$apply(function () {
-                hide();
-              });
+              hide();
+              if (!$rootScope.$$phase) {
+                $rootScope.$digest();
+              }
             }
 
             // Show the tooltip popup element.