**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.
switch (effectfulFiber.tag) {
      case ClassComponent:
        const instance = effectfulFiber.stateNode;
        const info: HandleErrorInfo = {
          componentStack: capturedError.componentStack,
        };
         
         
        instance.componentDidCatch(capturedError.error, info);
        return;
      case HostRoot:
        if (firstUncaughtError === null) {
           
           
           
          firstUncaughtError = capturedError.error;
        }
        return;
      default:
        invariant(
          false,
          'Invalid type of work. This error is likely caused by a bug in ' +
            'React. Please file an issue.',
        );
    }
  }
  function unwindContexts(from: Fiber, to: Fiber) {
    let node = from;
    while (node !== null) {
      switch (node.tag) {