svg attributes

format incorrectly changes camelcase attributes from SVG elements (and likely MathML etc.) to lowercase, e.g. `viewBox` becomes `viewbox`.

Replication:

```
import format from 'diffable-html'

console.log(format(''))
```

This is probably not an issue for comparing snapshots, but if you actually want the output of format to be correct, it's kind of important.