Inline SVG attributes get lowercased

Hi there,

the optimizer does not seem to respect the XML Namespace. As SVG is XML, and thus case sensitive (opposed to HTML, which is case insensitive), the [SVG viewBox attribute](https://www.w3.org/TR/SVG2/coords.html#ViewBoxAttribute) gets lowercased by the amp-optimizer. Browsers seem to ignore the problem, so this is more a nitpick.

See the `viewBox` attribute here:

Before:
```svg

```

After optimization:
```svg

```

The w3c validator complains about the lowercased `viewbox` attribute when validated as SVG 1.1:

No source file or test is named in the issue. Reproduce the reported inline SVG optimization using the before-and-after example, then locate the optimizer's attribute-name handling and add coverage showing that viewBox is preserved and the optimized SVG passes SVG validation.

Written by the indexing model from the issue text.