**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: string,
    props: Props,
  ) {
    warnForInsertedHydratedElement(parentInstance, type, props);
  },
  didNotFindHydratableTextInstance(
    parentInstance: Instance | Container,
    text: string,
  ) {
    warnForInsertedHydratedText(parentInstance, text);
  },
  scheduleDeferredCallback: ReactDOMFrameScheduling.rIC,
  useSyncScheduling: !ReactDOMFeatureFlags.fiberAsyncScheduling,
});
ReactGenericBatching.injection.injectFiberBatchedUpdates(
  DOMRenderer.batchedUpdates,
);
var warnedAboutHydrateAPI = false;
function renderSubtreeIntoContainer(
  parentComponent: ?React$Component<any, any>,
  children: ReactNodeList,
  container: DOMContainer,
  forceHydrate: boolean,
  callback: ?Function,
) {
  invariant(
    isValidContainer(container),
    'Target container is not a DOM element.',
  );
  if (__DEV__) {