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.

Start by running the reproduction in the issue and trace the formatter's handling of SVG attributes. Check how camelCase names such as viewBox are normalized, including whether the same behavior affects MathML. Done means formatted SVG preserves the correct attribute casing.

Written by the indexing model from the issue text.