**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.
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;
            
            handleCommitPhaseErrors();
            
            if (
              nextPriorityLevel === NoWork ||
              nextPriorityLevel > minPriorityLevel ||
              nextPriorityLevel > TaskPriority
            ) {
              
              break;
            }
          }
        }
      } else if (deadline !== null) {
        
        while (nextUnitOfWork !== null && !deadlineHasExpired) {
          if (deadline.timeRemaining() > timeHeuristicForUnitOfWork) {
            nextUnitOfWork = performUnitOfWork(nextUnitOfWork);