**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.
render() {
            return <div ref="foo" />;
          }
        }
        const e = await render(<RefComponent />);
        expect(e.getAttribute('ref')).toBe(null);
      });
      itRenders('no children attribute', async render => {
        const e = await render(React.createElement('div', {}, 'foo'));
        expect(e.getAttribute('children')).toBe(null);
      });
      itRenders('no key attribute', async render => {
        const e = await render(<div key="foo" />);
        expect(e.getAttribute('key')).toBe(null);
      });
      itRenders('no dangerouslySetInnerHTML attribute', async render => {
        const e = await render(
          <div dangerouslySetInnerHTML={{__html: '<foo />'}} />,
        );
        expect(e.getAttribute('dangerouslySetInnerHTML')).toBe(null);
      });
    });
    describe('inline styles', function() {
      itRenders('simple styles', async render => {
        const e = await render(<div style={{color: 'red', width: '30px'}} />);
        expect(e.style.color).toBe('red');