Popup's _updateContent method is used twice within onAdd

```
onAdd: function (map) {
    this._map = map;

    if (!this._container) {
        this._initLayout();
    }
    this._updateContent();
    ...
    this.update();
    ...

update: function () {
    if (!this._map) { return; }

    this._container.style.visibility = 'hidden';

    this._updateContent();
    ...
```

What purpose of that?
