'clipPathUnits' svg attribute is getting converted to 'clip-pathunits'

Input:
```html
<svg>
  <clipPath id="hexagon" clipPathUnits="objectBoundingBox">
    <polygon points="0.5 0, 1 0.25, 1 0.75, 0.5 1, 0 0.75, 0 0.25" />
  </clipPath>
</svg>
```
output:
```html
<svg>
  <clipPath id="hexagon" clip-pathunits="objectBoundingBox">
    <polygon points="0.5 0, 1 0.25, 1 0.75, 0.5 1, 0 0.75, 0 0.25" />
  </clipPath>
</svg>
```