**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.
return fn(a);
    } finally {
      isBatchingUpdates = previousIsBatchingUpdates;
       
       
      if (!isPerformingWork && !isBatchingUpdates) {
        performWork(TaskPriority, null);
      }
    }
  }
  function unbatchedUpdates<A>(fn: () => A): A {
    const previousIsUnbatchingUpdates = isUnbatchingUpdates;
    const previousIsBatchingUpdates = isBatchingUpdates;
     
    isUnbatchingUpdates = isBatchingUpdates;
    isBatchingUpdates = false;
    try {
      return fn();
    } finally {
      isBatchingUpdates = previousIsBatchingUpdates;
      isUnbatchingUpdates = previousIsUnbatchingUpdates;
    }
  }
  function flushSync<A>(batch: () => A): A {
    const previousIsBatchingUpdates = isBatchingUpdates;
    const previousPriorityContext = priorityContext;
    isBatchingUpdates = true;
    priorityContext = SynchronousPriority;
    try {
      return batch();