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>
```
d53eab8197
