horizontalBarPlot._updateXDomainer() calls super()

So does `verticalBarPlot._updateYDomainer()`. There wasn't a good reason we weren't calling `super`, I probably just forgot when I made each function.

Close #606.
That check is performed before we call `this._addBaseline`:

```typescript
    public _updateXDomainer() {
      super._updateXDomainer();
      var domainer = this._getDomainer(this.xScale);
      if (domainer != null) {
        this._addBaseline(domainer);
        this.xScale._autoDomainIfAutomaticMode();
      }
      return this;
    }
```

`this._getDomainer()` will return `null` if it's user set.