**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.
}
      });
      itRenders('null and false children together as blank', async render => {
        const e = await render(<div>{false}{null}foo{null}{false}</div>);
        if (ReactDOMFeatureFlags.useFiber) {
          
          expect(e.childNodes.length).toBe(1);
          expectTextNode(e.childNodes[0], 'foo');
        } else {
          
          expect(e.childNodes.length).toBe(3);
          expectTextNode(e.childNodes[0], 'foo');
        }
      });
      itRenders('only null and false children as blank', async render => {
        const e = await render(<div>{false}{null}{null}{false}</div>);
        expect(e.childNodes.length).toBe(0);
      });
    });
    describe('elements with implicit namespaces', function() {
      itRenders('an svg element', async render => {
        const e = await render(<svg />);
        expect(e.childNodes.length).toBe(0);
        expect(e.tagName).toBe('svg');
        expect(e.namespaceURI).toBe('http:www.w3.org/2000/svg');
      });
      itRenders('svg child element', async render => {
        let e = await render(
          <svg><image xlinkHref="http:i.imgur.com/w7GCRPb.png" /></svg>,
        );