**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.
var nextCoroutine = (workInProgress.pendingProps: null | ReactCoroutine);
    if (hasContextChanged()) {
      
      
      if (nextCoroutine === null) {
        nextCoroutine = current && current.memoizedProps;
        invariant(
          nextCoroutine !== null,
          'We should always have pending or current props. This error is ' +
            'likely caused by a bug in React. Please file an issue.',
        );
      }
    } else if (
      nextCoroutine === null ||
      workInProgress.memoizedProps === nextCoroutine
    ) {
      nextCoroutine = workInProgress.memoizedProps;
      
      
      
    }
    const nextChildren = nextCoroutine.children;
    const priorityLevel = workInProgress.pendingWorkPriority;