fix(ionView): do not set navbar title if no title attr set

Fixes #915

BREAKING CHANGE: Before, if you did not have a `title` attribute set on your
ion-view, it would transition into that view and erase the navbar's current
title.

Now, if your ion-view does not have a `title` attribute set, the new
view will be transitioned in, but there will be no title change.

If you wish to have a blank title on your new view, you must now
explicitly set your `ion-view`'s title attribute to an empty string.

To migrate your code, change from this:

```html
<ion-view></ion-view>
```

To this:

```html
<ion-view title=""></ion-view>
```

js/ext/angular/src/directive/ionicContent.js

@@ -48,7 +48,7 @@ angular.module('ionic.ui.content', ['ionic.ui.scroll'])
  * with {@link ionic.service:$ionicScrollDelegate}.
  * @param {boolean=} padding Whether to add padding to the content.
  * of the content.  Defaults to true on iOS, false on Android.
- * @param {boolean=} scroll Whether to allow scrolling of content.  Defaults to true. Note: scroll="false" removes the .scroll child element on element compilation, not on scope change
+ * @param {boolean=} scroll Whether to allow scrolling of content.  Defaults to true.
  * @param {boolean=} overflow-scroll Whether to use overflow-scrolling instead of
  * Ionic scroll.
  * @param {boolean=} has-bouncing Whether to allow scrolling to bounce past the edges

js/ext/angular/src/directive/ionicViewState.js

@@ -48,15 +48,20 @@ angular.module('ionic.ui.viewState', ['ionic.service.view', 'ionic.service.gestu
         if (!navBarCtrl) {
           return;
         }
-        var initialTitle = $attr.title;
-        navBarCtrl.changeTitle(initialTitle, $scope.$navDirection);
 
-        // watch for changes in the title, don't set initial value as changeTitle does that
-        $attr.$observe('title', function(val, oldVal) {
-          if (val !== initialTitle) {
-            navBarCtrl.setTitle(val);
-          }
-        });
+        if (angular.isDefined($attr.title)) {
+
+          var initialTitle = $attr.title;
+          navBarCtrl.changeTitle(initialTitle, $scope.$navDirection);
+
+          // watch for changes in the title, don't set initial value as changeTitle does that
+          $attr.$observe('title', function(val, oldVal) {
+            if (val !== initialTitle) {
+              navBarCtrl.setTitle(val);
+            }
+          });
+
+        }
 
         $scope.$watch($attr.hideBackButton, function(value) {
           // Should we hide a back button when this tab is shown

js/ext/angular/test/directive/ionicView.unit.js

@@ -33,7 +33,17 @@ describe('ionView directive', function() {
     expect(el.html()).toBe('<b>some</b> html');
   });
 
-  it('should changeTitle with a navDirection', function() {
+  it('should not changeTitle with undefined title attr', function() {
+    var el = setup();
+    expect(el.controller('ionNavBar').changeTitle).not.toHaveBeenCalled();
+  });
+
+  it('should changeTitle with blank if title attr is blank', function() {
+    var el = setup('title=""', {$navDirection: 'someDirection'});
+    expect(el.controller('ionNavBar').changeTitle).toHaveBeenCalledWith('', 'someDirection');
+  });
+
+  it('should changeTitle with a navDirection if title set', function() {
     var el = setup('title="Hi, {{1}}!"', {$navDirection: 'foo'});
     expect(el.controller('ionNavBar').changeTitle).toHaveBeenCalledWith('Hi, 1!', 'foo');
   });