**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.
const Component = props => <div>{props.children}</div>;
        const e = await render(
          <Component>
            <Component>
              <Component /><Component />
            </Component>
            <Component>
              <Component /><Component />
            </Component>
          </Component>,
        );
        expect(e.tagName).toBe('DIV');
        expect(e.childNodes.length).toBe(2);
        for (var i = 0; i < 2; i++) {
          var child = e.childNodes[i];
          expect(child.tagName).toBe('DIV');
          expect(child.childNodes.length).toBe(2);
          for (var j = 0; j < 2; j++) {
            var grandchild = child.childNodes[j];
            expect(grandchild.tagName).toBe('DIV');
            expect(grandchild.childNodes.length).toBe(0);
          }
        }
      });
      itRenders('a div with a child', async render => {
        const e = await render(<div id="parent"><div id="child" /></div>);
        expect(e.id).toBe('parent');
        expect(e.childNodes.length).toBe(1);
        expect(e.childNodes[0].id).toBe('child');
        expect(e.childNodes[0].childNodes.length).toBe(0);