**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.
var ReactFiberClassComponent = require('ReactFiberClassComponent');
var {ReactCurrentOwner} = require('ReactGlobalSharedState');
var invariant = require('fbjs/lib/invariant');
if (__DEV__) {
  var ReactDebugCurrentFiber = require('ReactDebugCurrentFiber');
  var {cancelWorkTimer} = require('ReactDebugFiberPerf');
  var warning = require('fbjs/lib/warning');
  var warnedAboutStatelessRefs = {};
}
module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  config: HostConfig<T, P, I, TI, PI, C, CX, PL>,
  hostContext: HostContext<C, CX>,
  hydrationContext: HydrationContext<C>,
  scheduleUpdate: (fiber: Fiber, priorityLevel: PriorityLevel) => void,
  getPriorityContext: (fiber: Fiber, forceAsync: boolean) => PriorityLevel,
) {
  const {
    shouldSetTextContent,
    useSyncScheduling,
    shouldDeprioritizeSubtree,
  } = config;
  const {pushHostContext, pushHostContainer} = hostContext;
  const {
    enterHydrationState,
    resetHydrationState,
    tryToClaimNextHydratableInstance,
  } = hydrationContext;
  const {
    adoptClassInstance,
    constructClassInstance,