fix(tooltip): respect animate attribute

Previously, it was only possible to set animation to false with an empty
string and options.

Author: chrisirhc

src/tooltip/test/tooltip.spec.js

@@ -45,6 +45,10 @@ describe('tooltip', function() {
     expect( elmScope.tt_isOpen ).toBe( false );
   }));
 
+  it('should not animate on animation set to false', inject(function() {
+    expect( elmScope.tt_animation ).toBe( false );
+  }));
+
   it('should have default placement of "top"', inject(function() {
     elm.trigger( 'mouseenter' );
     expect( elmScope.tt_placement ).toBe( "top" );

src/tooltip/tooltip.js

@@ -262,7 +262,8 @@ angular.module( 'ui.bootstrap.tooltip', [ 'ui.bootstrap.position', 'ui.bootstrap
           });
 
           attrs.$observe(prefix + 'Animation', function (val) {
-            scope.tt_animation = angular.isDefined(val) ? !!val : options.animation;
+            var shouldAnimate = val !== 'false';
+            scope.tt_animation = angular.isDefined(val) ? shouldAnimate : options.animation;
           });
 
           attrs.$observe( prefix+'PopupDelay', function ( val ) {