**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.
flushSync,
    deferredUpdates,
  } = ReactFiberScheduler(config);
  function scheduleTopLevelUpdate(
    current: Fiber,
    element: ReactNodeList,
    callback: ?Function,
  ) {
    if (__DEV__) {
      if (
        ReactDebugCurrentFiber.phase === 'render' &&
        ReactDebugCurrentFiber.current !== null
      ) {
        warning(
          false,
          'Render methods should be a pure function of props and state; ' +
            'triggering nested component updates from render is not allowed. ' +
            'If necessary, trigger nested updates in componentDidUpdate.\n\n' +
            'Check the render method of %s.',
          getComponentName(ReactDebugCurrentFiber.current) || 'Unknown',
        );
      }
    }
    
    
    
    const forceAsync =
      ReactFeatureFlags.enableAsyncSubtreeAPI &&
      element != null &&
      element.type != null &&