**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.
callback: ?Function,
  ): void,
  batchedUpdates<A>(fn: () => A): A,
  unbatchedUpdates<A>(fn: () => A): A,
  flushSync<A>(fn: () => A): A,
  deferredUpdates<A>(fn: () => A): A,
  
  getPublicRootInstance(
    container: OpaqueRoot,
  ): React$Component<any, any> | TI | I | null,
  
  findHostInstance(component: Fiber): I | TI | null,
  
  findHostInstanceWithNoPortals(component: Fiber): I | TI | null,
};
getContextForSubtree._injectFiber(function(fiber: Fiber) {
  const parentContext = findCurrentUnmaskedContext(fiber);
  return isContextProvider(fiber)
    ? processChildContext(fiber, parentContext, false)
    : parentContext;
});
module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  config: HostConfig<T, P, I, TI, PI, C, CX, PL>,
): Reconciler<C, I, TI> {
  var {getPublicInstance} = config;
  var {
    scheduleUpdate,
    getPriorityContext,
    batchedUpdates,
    unbatchedUpdates,