**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.
},
            };
          },
        };
        let e = await render(threeDivIterable);
        let parent = e.parentNode;
        expect(parent.childNodes.length).toBe(3);
        expect(parent.childNodes[0].tagName).toBe('DIV');
        expect(parent.childNodes[1].tagName).toBe('DIV');
        expect(parent.childNodes[2].tagName).toBe('DIV');
      });
      itRenders('emptyish values', async render => {
        let e = await render(0);
        expect(e.nodeType).toBe(TEXT_NODE_TYPE);
        expect(e.nodeValue).toMatch('0');
        
        
        expect((await render(<div>{''}</div>)).textContent).toBe('');
        expect(await render([])).toBe(null);
        expect(await render(false)).toBe(null);
        expect(await render(true)).toBe(null);
        expect(await render(undefined)).toBe(null);
        expect(await render([[[false]], undefined])).toBe(null);
      });
    }
  });
  describe('property to attribute mapping', function() {
    describe('string properties', function() {
      itRenders('simple numbers', async render => {
        const e = await render(<div width={30} />);