**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.
workLoop(minPriorityLevel, deadline);
  }
  function performWork(
    minPriorityLevel: PriorityLevel,
    deadline: Deadline | null,
  ) {
    if (__DEV__) {
      startWorkLoopTimer();
    }
    invariant(
      !isPerformingWork,
      'performWork was called recursively. This error is likely caused ' +
        'by a bug in React. Please file an issue.',
    );
    isPerformingWork = true;
     The priority context changes during the render phase. We'll need to
     reset it at the end.
    const previousPriorityContext = priorityContext;
    let didError = false;
    let error = null;
    if (__DEV__) {
      invokeGuardedCallback(null, workLoop, null, minPriorityLevel, deadline);
      if (hasCaughtError()) {
        didError = true;
        error = clearCaughtError();
      }
    } else {
      try {
        workLoop(minPriorityLevel, deadline);