fix(tooltip): update tooltip content when empty

Closes #1348

Author: bekos

src/tooltip/test/tooltip2.spec.js

@@ -70,6 +70,17 @@ describe('tooltip directive', function () {
       $timeout.flush();
       expect(fragment).not.toHaveOpenTooltips();
     });
+
+    it('should update tooltip when its content becomes empty', function () {
+      $rootScope.content = 'some text';
+      var fragment = compileTooltip('<span tooltip="{{ content }}"></span>');
+
+      $rootScope.content = '';
+      $rootScope.$digest();
+
+      fragment.find('span').trigger( 'mouseenter' );
+      expect(fragment).not.toHaveOpenTooltips();
+    });
   });
 
   describe('option by option', function () {

src/tooltip/tooltip.js

@@ -247,12 +247,10 @@ angular.module( 'ui.bootstrap.tooltip', [ 'ui.bootstrap.position', 'ui.bootstrap
            * Observe the relevant attributes.
            */
           attrs.$observe( type, function ( val ) {
-            if (val) {
-              scope.tt_content = val;
-            } else {
-              if ( scope.tt_isOpen ) {
-                hide();
-              }
+            scope.tt_content = val;
+
+            if (!val && scope.tt_isOpen ) {
+              hide();
             }
           });