**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.
| (Element & {
    _reactRootContainer: ?Object,
  })
  | (Document & {
    _reactRootContainer: ?Object,
  });
type Container = Element | Document;
type Props = {
  autoFocus?: boolean,
  children?: mixed,
  hidden?: boolean,
};
type Instance = Element;
type TextInstance = Text;
type HostContextDev = {
  namespace: string,
  ancestorInfo: mixed,
};
type HostContextProd = string;
type HostContext = HostContextDev | HostContextProd;
let eventsEnabled: ?boolean = null;
let selectionInformation: ?mixed = null;
/**
 * True if the supplied DOM node is a valid node element.
 *
 * @param {?DOMElement} node The candidate DOM node.
 * @return {boolean} True if the DOM is a valid DOM node.
 * @internal
 */
function isValidContainer(node) {