Fix exiting namespace when it's the last child (#223)

Fixes https://github.com/tildeio/htmlbars/issues/223 (which came from https://github.com/emberjs/ember.js/issues/9971 )
It doesn't really matter performance-wise, should be cheap, but it adds unnecessary work to the template. For example:

```javascript
setNamespace(svgUrl);
createElement('svg');
setNamespace(null); // If this is the end of a template, it would be nice to avoid this reset
```