**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.
isCommitting = false;
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    if (typeof onCommitRoot === 'function') {
      onCommitRoot(finishedWork.stateNode);
    }
    if (__DEV__ && ReactFiberInstrumentation.debugTool) {
      ReactFiberInstrumentation.debugTool.onCommitWork(finishedWork);
    }
    
    
    if (commitPhaseBoundaries) {
      commitPhaseBoundaries.forEach(scheduleErrorRecovery);
      commitPhaseBoundaries = null;
    }
    
    
    resetNextUnitOfWork();
  }
  function resetWorkPriority(
    workInProgress: Fiber,
    renderPriority: PriorityLevel,
  ) {
    if (
      workInProgress.pendingWorkPriority !== NoWork &&
      workInProgress.pendingWorkPriority > renderPriority
    ) {