**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.
type !== 'body' &&
        !shouldSetTextContent(type, fiber.memoizedProps))
    ) {
      let nextInstance = nextHydratableInstance;
      while (nextInstance) {
        deleteHydratableInstance(fiber, nextInstance);
        nextInstance = getNextHydratableSibling(nextInstance);
      }
    }
    popToNextHostParent(fiber);
    nextHydratableInstance = hydrationParentFiber
      ? getNextHydratableSibling(fiber.stateNode)
      : null;
    return true;
  }
  function resetHydrationState() {
    hydrationParentFiber = null;
    nextHydratableInstance = null;
    isHydrating = false;
  }
  return {
    enterHydrationState,
    resetHydrationState,
    tryToClaimNextHydratableInstance,
    prepareToHydrateHostInstance,
    prepareToHydrateHostTextInstance,
    popHydrationState,
  };
};