**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**
I got this warning: `Warning: Extra attributes from the server: preserveAspectRatio,viewBox` during client side hydratation when my react tree contains mixed case attributes such as these svg ones.

**What is the expected behavior?**

No warnings because there are no difference between server and client attributes.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

**React 16.0.0-rc.2**
This worked on 15.*

This seems to happen because the attribute name is put to lowercase here: https://github.com/facebook/react/blob/6552519a211c3e43e3f6cf5b2237c90ae515d423/src/renderers/dom/fiber/ReactDOMFiberComponent.js#L901
but not in the set here:
https://github.com/facebook/react/blob/6552519a211c3e43e3f6cf5b2237c90ae515d423/src/renderers/dom/fiber/ReactDOMFiberComponent.js#L915
As the set removal is done with the lowercased attribute:
https://github.com/facebook/react/blob/6552519a211c3e43e3f6cf5b2237c90ae515d423/src/renderers/dom/fiber/ReactDOMFiberComponent.js#L1011
it stays in the `extraAttributeNames` set and I got wrongly the previous warning.
constructor(options) {
    super(options);
    this.buffer = '';
  }
  _write(chunk, encoding, cb) {
    this.buffer += chunk;
    cb();
  }
}
async function renderIntoStream(reactElement, errorCount = 0) {
  return await expectErrors(
    () =>
      new Promise(resolve => {
        let writable = new DrainWritable();
        ReactDOMServer.renderToNodeStream(reactElement).pipe(writable);
        writable.on('finish', () => resolve(writable.buffer));
      }),
    errorCount,
  );
}
async function streamRender(reactElement, errorCount = 0) {
  const markup = await renderIntoStream(reactElement, errorCount);
  var domElement = document.createElement('div');
  domElement.innerHTML = markup;
  return domElement.firstChild;
}
const clientCleanRender = (element, errorCount = 0) => {
  const div = document.createElement('div');