**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.
if (__DEV__) {
      switch (returnFiber.tag) {
        case HostRoot:
          didNotHydrateInstance(returnFiber.stateNode.containerInfo, instance);
          break;
        case HostComponent:
          didNotHydrateInstance(returnFiber.stateNode, instance);
          break;
      }
    }
    const childToDelete = createFiberFromHostInstanceForDeletion();
    childToDelete.stateNode = instance;
    childToDelete.return = returnFiber;
    childToDelete.effectTag = Deletion;
    
    
    
    
    
    if (returnFiber.lastEffect !== null) {
      returnFiber.lastEffect.nextEffect = childToDelete;
      returnFiber.lastEffect = childToDelete;
    } else {
      returnFiber.firstEffect = returnFiber.lastEffect = childToDelete;
    }
  }
  function insertNonHydratedInstance(returnFiber: Fiber, fiber: Fiber) {