**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.
shouldContinue = true;
        node.pendingWorkPriority = priorityLevel;
      }
      if (node.alternate !== null) {
        if (
          node.alternate.pendingWorkPriority === NoWork ||
          node.alternate.pendingWorkPriority > priorityLevel
        ) {
           
          shouldContinue = true;
          node.alternate.pendingWorkPriority = priorityLevel;
        }
      }
      if (node.return === null) {
        if (node.tag === HostRoot) {
          const root: FiberRoot = (node.stateNode: any);
          scheduleRoot(root, priorityLevel);
          if (!isPerformingWork) {
            switch (priorityLevel) {
              case SynchronousPriority:
                 
                if (isUnbatchingUpdates) {
                   
                   
                   
                  performWork(SynchronousPriority, null);
                } else {
                   
                  performWork(TaskPriority, null);