**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.
shouldSetTextContent(type: T, props: P): boolean,
  resetTextContent(instance: I): void,
  shouldDeprioritizeSubtree(type: T, props: P): boolean,
  createTextInstance(
    text: string,
    rootContainerInstance: C,
    hostContext: CX,
    internalInstanceHandle: OpaqueHandle,
  ): TI,
  commitTextUpdate(textInstance: TI, oldText: string, newText: string): void,
  appendChild(parentInstance: I, child: I | TI): void,
  appendChildToContainer(container: C, child: I | TI): void,
  insertBefore(parentInstance: I, child: I | TI, beforeChild: I | TI): void,
  insertInContainerBefore(
    container: C,
    child: I | TI,
    beforeChild: I | TI,
  ): void,
  removeChild(parentInstance: I, child: I | TI): void,
  removeChildFromContainer(container: C, child: I | TI): void,
  scheduleDeferredCallback(
    callback: (deadline: Deadline) => void,
  ): number | void,
  prepareForCommit(): void,
  resetAfterCommit(): void,
  
  canHydrateInstance?: (instance: I | TI, type: T, props: P) => boolean,
  canHydrateTextInstance?: (instance: I | TI, text: string) => boolean,
  getNextHydratableSibling?: (instance: I | TI) => null | I | TI,
  getFirstHydratableChild?: (parentInstance: I | C) => null | I | TI,