**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.
if (
      capturedErrors !== null &&
      capturedErrors.size > 0 &&
      nextPriorityLevel === TaskPriority
    ) {
      while (nextUnitOfWork !== null) {
        if (hasCapturedError(nextUnitOfWork)) {
          
          nextUnitOfWork = performFailedUnitOfWork(nextUnitOfWork);
        } else {
          nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
        }
        if (nextUnitOfWork === null) {
          invariant(
            pendingCommit !== null,
            'Should have a pending commit. This error is likely caused by ' +
              'a bug in React. Please file an issue.',
          );
          
          priorityContext = TaskPriority;
          commitAllWork(pendingCommit);
          priorityContext = nextPriorityLevel;
          if (
            capturedErrors === null ||
            capturedErrors.size === 0 ||
            nextPriorityLevel !== TaskPriority
          ) {