**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.
current,
      workInProgress,
      shouldUpdate,
      hasContext,
    );
  }
  function finishClassComponent(
    current: Fiber | null,
    workInProgress: Fiber,
    shouldUpdate: boolean,
    hasContext: boolean,
  ) {
    
    markRef(current, workInProgress);
    if (!shouldUpdate) {
      
      if (hasContext) {
        invalidateContextProvider(workInProgress, false);
      }
      return bailoutOnAlreadyFinishedWork(current, workInProgress);
    }
    const instance = workInProgress.stateNode;
    
    ReactCurrentOwner.current = workInProgress;
    let nextChildren;
    if (__DEV__) {
      ReactDebugCurrentFiber.setCurrentFiber(workInProgress, 'render');
      nextChildren = instance.render();
      ReactDebugCurrentFiber.setCurrentFiber(workInProgress, null);
    } else {