**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.
e = e.firstChild;
        expect(e.childNodes.length).toBe(0);
        expect(e.tagName).toBe('image');
        expect(e.namespaceURI).toBe('http:www.w3.org/2000/svg');
        expect(e.getAttributeNS('http:www.w3.org/1999/xlink', 'href')).toBe(
          'http:i.imgur.com/w7GCRPb.png',
        );
      });
      itRenders('svg child element with a badly cased alias', async render => {
        let e = await render(
          <svg><image xlinkhref="http:i.imgur.com/w7GCRPb.png" /></svg>,
          1,
        );
        e = e.firstChild;
        expect(e.hasAttributeNS('http:www.w3.org/1999/xlink', 'href')).toBe(
          false,
        );
        expect(e.getAttribute('xlinkhref')).toBe(
          'http:i.imgur.com/w7GCRPb.png',
        );
      });
      itRenders('a math element', async render => {
        const e = await render(<math />);
        expect(e.childNodes.length).toBe(0);
        expect(e.tagName).toBe('math');
        expect(e.namespaceURI).toBe('http:www.w3.org/1998/Math/MathML');
      });
    });