**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.
const NESTED_UPDATE_LIMIT = 1000;
  let nestedUpdateCount: number = 0;
  let nextRenderedTree: FiberRoot | null = null;
  function resetContextStack() {
    
    reset();
    
    resetContext();
    resetHostContainer();
  }
   resetNextUnitOfWork mutates the current priority context. It is reset after
   after the workLoop exits, so never call resetNextUnitOfWork from outside
   the work loop.
  function resetNextUnitOfWork() {
    
    while (
      nextScheduledRoot !== null &&
      nextScheduledRoot.current.pendingWorkPriority === NoWork
    ) {
      
      nextScheduledRoot.isScheduled = false;
      
      
      const next = nextScheduledRoot.nextScheduledRoot;
      nextScheduledRoot.nextScheduledRoot = null;
      
      if (nextScheduledRoot === lastScheduledRoot) {
        nextScheduledRoot = null;
        lastScheduledRoot = null;
        nextPriorityLevel = NoWork;