xlink:href becomes xlink::href

#310 fixes `xlinkHref` attributes but breaks `xlink:href` by adding another colon:

```js
> require('preact-render-to-string').renderToString({constructor: undefined, type: 'use', props: {'xlink:href': 'hi'}})
'<use xlink::href="hi"></use>'
```

Perhaps `NAMESPACE_REPLACE_REGEX` should just be `/^(xlink|xmlns|xml)([A-Z])/`?