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
Initially I wanted to do `String(data)` (and I agree it's the way it should be done),  but in function above you do `data + ""`, I didn't want to break _convention_. Should I then update both places?