**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.
container: Container,
    child: Instance | TextInstance,
  ): void {
    if (container.nodeType === COMMENT_NODE) {
      (container.parentNode: any).removeChild(child);
    } else {
      container.removeChild(child);
    }
  },
  canHydrateInstance(
    instance: Instance | TextInstance,
    type: string,
    props: Props,
  ): boolean {
    return (
      instance.nodeType === ELEMENT_NODE &&
      type === instance.nodeName.toLowerCase()
    );
  },
  canHydrateTextInstance(
    instance: Instance | TextInstance,
    text: string,
  ): boolean {
    if (text === '') {
      
      return false;
    }
    return instance.nodeType === TEXT_NODE;
  },
  getNextHydratableSibling(