**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.
} = require('ReactFiberContext');
var {createFiberRoot} = require('ReactFiberRoot');
var ReactFiberScheduler = require('ReactFiberScheduler');
var {HostComponent} = require('ReactTypeOfWork');
if (__DEV__) {
  var warning = require('fbjs/lib/warning');
  var ReactFiberInstrumentation = require('ReactFiberInstrumentation');
  var ReactDebugCurrentFiber = require('ReactDebugCurrentFiber');
  var getComponentName = require('getComponentName');
}
var {
  findCurrentHostFiber,
  findCurrentHostFiberWithNoPortals,
} = require('ReactFiberTreeReflection');
var getContextForSubtree = require('getContextForSubtree');
export type Deadline = {
  timeRemaining: () => number,
};
type OpaqueHandle = Fiber;
type OpaqueRoot = FiberRoot;
export type HostConfig<T, P, I, TI, PI, C, CX, PL> = {
  getRootHostContext(rootContainerInstance: C): CX,
  getChildHostContext(parentHostContext: CX, type: T, instance: C): CX,
  getPublicInstance(instance: I | TI): PI,
  createInstance(
    type: T,
    props: P,
    rootContainerInstance: C,
    hostContext: CX,
    internalInstanceHandle: OpaqueHandle,