**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.
(hydrationParentFiber: any),
        nextHydratableInstance,
      );
    }
    fiber.stateNode = nextInstance;
    hydrationParentFiber = fiber;
    nextHydratableInstance = getFirstHydratableChild(nextInstance);
  }
  function prepareToHydrateHostInstance(
    fiber: Fiber,
    rootContainerInstance: C,
  ): boolean {
    const instance: I = fiber.stateNode;
    const updatePayload = hydrateInstance(
      instance,
      fiber.type,
      fiber.memoizedProps,
      rootContainerInstance,
      fiber,
    );
    
    fiber.updateQueue = (updatePayload: any);
    
    
    if (updatePayload !== null) {
      return true;
    }
    return false;
  }
  function prepareToHydrateHostTextInstance(fiber: Fiber): boolean {