**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.
(fiber.alternate !== null && capturedErrors.has(fiber.alternate)))
    );
  }
  function isFailedBoundary(fiber: Fiber): boolean {
     
     
    return (
      failedBoundaries !== null &&
      (failedBoundaries.has(fiber) ||
        (fiber.alternate !== null && failedBoundaries.has(fiber.alternate)))
    );
  }
  function commitErrorHandling(effectfulFiber: Fiber) {
    let capturedError;
    if (capturedErrors !== null) {
      capturedError = capturedErrors.get(effectfulFiber);
      capturedErrors.delete(effectfulFiber);
      if (capturedError == null) {
        if (effectfulFiber.alternate !== null) {
          effectfulFiber = effectfulFiber.alternate;
          capturedError = capturedErrors.get(effectfulFiber);
          capturedErrors.delete(effectfulFiber);
        }
      }
    }
    invariant(
      capturedError != null,
      'No error for given unit of work. This error is likely caused by a ' +
        'bug in React. Please file an issue.',
    );