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
That would be lovely @medikoo, thank you :). Sorry for our code being inconsistent beforehand. And sorry for the delay in responding to a pretty simple PR; I'm on vacation with spotty internet access for the next few days.