**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.
popHydrationState(fiber: Fiber): boolean,
};
module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  config: HostConfig<T, P, I, TI, PI, C, CX, PL>,
): HydrationContext<C> {
  const {
    shouldSetTextContent,
    canHydrateInstance,
    canHydrateTextInstance,
    getNextHydratableSibling,
    getFirstHydratableChild,
    hydrateInstance,
    hydrateTextInstance,
    didNotHydrateInstance,
    didNotFindHydratableInstance,
    didNotFindHydratableTextInstance,
  } = config;
  
  if (
    !(canHydrateInstance &&
      canHydrateTextInstance &&
      getNextHydratableSibling &&
      getFirstHydratableChild &&
      hydrateInstance &&
      hydrateTextInstance &&
      didNotHydrateInstance &&
      didNotFindHydratableInstance &&
      didNotFindHydratableTextInstance)
  ) {
    return {