**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.
hostContext: HostContext<C, CX>,
  hydrationContext: HydrationContext<C>,
) {
  const {
    createInstance,
    createTextInstance,
    appendInitialChild,
    finalizeInitialChildren,
    prepareUpdate,
  } = config;
  const {
    getRootHostContainer,
    popHostContext,
    getHostContext,
    popHostContainer,
  } = hostContext;
  const {
    prepareToHydrateHostInstance,
    prepareToHydrateHostTextInstance,
    popHydrationState,
  } = hydrationContext;
  function markUpdate(workInProgress: Fiber) {
    
    
    workInProgress.effectTag |= Update;
  }
  function markRef(workInProgress: Fiber) {
    workInProgress.effectTag |= Ref;
  }
  function appendAllYields(yields: Array<mixed>, workInProgress: Fiber) {