**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.
return {
            render: function() {
              return <div>foo</div>;
            },
          };
        };
        checkFooDiv(await render(<FactoryComponent />));
      });
    });
    describe('component hierarchies', async function() {
      itRenders('single child hierarchies of components', async render => {
        const Component = props => <div>{props.children}</div>;
        let e = await render(
          <Component>
            <Component>
              <Component>
                <Component />
              </Component>
            </Component>
          </Component>,
        );
        for (var i = 0; i < 3; i++) {
          expect(e.tagName).toBe('DIV');
          expect(e.childNodes.length).toBe(1);
          e = e.firstChild;
        }
        expect(e.tagName).toBe('DIV');
        expect(e.childNodes.length).toBe(0);
      });
      itRenders('multi-child hierarchies of components', async render => {