**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.
workInProgress.tag = CoroutineHandlerPhase;
    
    
    var yields: Array<mixed> = [];
    appendAllYields(yields, workInProgress);
    var fn = coroutine.handler;
    var props = coroutine.props;
    var nextChildren = fn(props, yields);
    var currentFirstChild = current !== null ? current.child : null;
    // Inherit the priority of the returnFiber.
    const priority = workInProgress.pendingWorkPriority;
    workInProgress.child = reconcileChildFibers(
      workInProgress,
      currentFirstChild,
      nextChildren,
      priority,
    );
    return workInProgress.child;
  }
  function appendAllChildren(parent: I, workInProgress: Fiber) {
    
    
    let node = workInProgress.child;
    while (node !== null) {
      if (node.tag === HostComponent || node.tag === HostText) {
        appendInitialChild(parent, node.stateNode);
      } else if (node.tag === HostPortal) {