Render class instead of className

[MicheleBertoli]

preact-render-to-string always renders class (instead of className), which is pretty useful - especially for snapshot testing.

if (name==='className') {
  if (attributes['class']) continue;
  name = 'class';
}

(see https://github.com/developit/preact-render-to-string/blob/master/src/index.js#L118)

This diff implements the same functionality for preact-render-to-json.

[nathancahill]

Thanks!