**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.
hydrateInstance?: (
    instance: I,
    type: T,
    props: P,
    rootContainerInstance: C,
    internalInstanceHandle: OpaqueHandle,
  ) => null | PL,
  hydrateTextInstance?: (
    textInstance: TI,
    text: string,
    internalInstanceHandle: OpaqueHandle,
  ) => boolean,
  didNotHydrateInstance?: (parentInstance: I | C, instance: I | TI) => void,
  didNotFindHydratableInstance?: (
    parentInstance: I | C,
    type: T,
    props: P,
  ) => void,
  didNotFindHydratableTextInstance?: (
    parentInstance: I | C,
    text: string,
  ) => void,
  useSyncScheduling?: boolean,
};
export type Reconciler<C, I, TI> = {
  createContainer(containerInfo: C): OpaqueRoot,
  updateContainer(
    element: ReactNodeList,
    container: OpaqueRoot,
    parentComponent: ?React$Component<any, any>,