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.

首先运行 issue 中的复现步骤，并跟踪 formatter 对 SVG 属性的处理方式。检查 camelCase 名称（例如 viewBox）是如何被规范化的，包括相同的行为是否也会影响 MathML。完成标准是：格式化后的 SVG 保留正确的属性大小写。

由索引模型根据 Issue 内容生成。