**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;
    }
    
    let newPriority = getUpdatePriority(workInProgress);
    
    let child = workInProgress.child;
    while (child !== null) {
      
      newPriority = largerPriority(newPriority, child.pendingWorkPriority);
      child = child.sibling;
    }
    workInProgress.pendingWorkPriority = newPriority;
  }
  function completeUnitOfWork(workInProgress: Fiber): Fiber | null {
    while (true) {
      
      
      
      
      const current = workInProgress.alternate;
      const next = completeWork(current, workInProgress, nextPriorityLevel);
      const returnFiber = workInProgress.return;
      const siblingFiber = workInProgress.sibling;
      resetWorkPriority(workInProgress, nextPriorityLevel);
      if (next !== null) {
        if (__DEV__) {
          stopWorkTimer(workInProgress);
        }
        if (__DEV__ && ReactFiberInstrumentation.debugTool) {
          ReactFiberInstrumentation.debugTool.onCompleteWork(workInProgress);