fix(ngBind): use same string representation as $interpolate

Fixes #11716

BREAKING CHANGE:

`ngBind` now uses the same logic as $interpolate (i.e. {{myString}}) when
binding, which means values other than strings are now transformed as following:
- null / undefined become empty string
- with an object's custom toString() function, except if the object is a Date, Array, or Number
- otherwise with JSON.stringify

Previously, ngBind would use always use toString().

The following examples show the different output:
```js
$scope.myPlainObject = {a: 1, b: 2};
$scope.myCustomObject = {a: 1, b: 2, toString: function() {return 'a+b';}};
```

Plain Object:
```html
<!-- Before: -->
<span ng-bind="myPlainObject">[object Object]</span>

<!-- After: -->
<span ng-bind="myPlainObject">{"a":1,"b":2}</span>
```

Object with custom toString():

```html
<!-- Before: -->
<span ng-bind="myCustomObject">[object Object]</span>

<!-- After: -->
<span ng-bind="myCustomObject">a+b</span>
```

If you want the output of `toString()`, you can use it directly on the value in ngBind:

```html
<span ng-bind="myObject.toString()">[object Object]</span>
```