**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.
startCommitTimer();
    }
    pendingCommit = null;
    const root: FiberRoot = (finishedWork.stateNode: any);
    invariant(
      root.current !== finishedWork,
      'Cannot commit the same tree as before. This is probably a bug ' +
        'related to the return field. This error is likely caused by a bug ' +
        'in React. Please file an issue.',
    );
    if (
      nextPriorityLevel === SynchronousPriority ||
      nextPriorityLevel === TaskPriority
    ) {
      
      
      nestedUpdateCount++;
    }
     Reset this to null before calling lifecycles
    ReactCurrentOwner.current = null;
    let firstEffect;
    if (finishedWork.effectTag > PerformedWork) {
      
      
      
      
      if (finishedWork.lastEffect !== null) {
        finishedWork.lastEffect.nextEffect = finishedWork;
        firstEffect = finishedWork.firstEffect;
      } else {