Assure string normalization on set of text.data

Setting `data` on text node, doesn't normalize value to string

```javascript
textNode.data = 34;
typeof textNode.data; // 'number' (should be 'string')
```
This change fixes that
There is; `x + ""` will give the wrong answer for `{ valueOf() { return 42; }, toString() { return "s"; }`.