Good question. This is for consistent behaviour with #690 regarding property values that are not explicitly set by the user.

Case for #690:

User creates a layer:
```javascript
var layer = new ol.layer.Layer({
  source: ...
});
```
If default values are not set, then you get strange behaviour: `layer.getVisible()` returns `undefined` (which is logically false) but the layer is visible by default. Without explicitly setting a default value, you'll get undesired behaviour if you try to toggle a layer's visibility with
```javascript
layer.setVisible(!layer.getVisible());
```
The easy (and, I think, least bad) solution is to explicitly set the default values of properties.

This change applies the same logic to `rotation`, so that, for example:
```javascript
var view = new ol.View2D({});
view.setRotation(view.getRotation() + Math.PI / 4);
```
works as you'd expect.