fix(tooltip): do not toggle when isOpen is not boolean

- Coerce `isOpen` to boolean to do a proper comparison for firing
  `toggleTooltipBind`

Author: wesleycho

src/tooltip/tooltip.js

@@ -379,7 +379,8 @@ angular.module('ui.bootstrap.tooltip', ['ui.bootstrap.position', 'ui.bootstrap.s
 
             if (isOpenExp) {
               scope.$watch(isOpenExp, function(val) {
-                if (val !== ttScope.isOpen) {
+                /*jshint -W018 */
+                if (!val === ttScope.isOpen) {
                   toggleTooltipBind();
                 }
               });