**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.
itRenders('no className prop with null value', async render => {
        const e = await render(<div className={null} />);
        expect(e.hasAttribute('className')).toBe(false);
      });
      itRenders('badly cased className with a warning', async render => {
        const e = await render(<div classname="test" />, 1);
        expect(e.hasAttribute('class')).toBe(false);
        expect(e.hasAttribute('classname')).toBe(true);
      });
      itRenders(
        'className prop when given the alias with a warning',
        async render => {
          const e = await render(<div class="test" />, 1);
          expect(e.className).toBe('test');
        },
      );
      itRenders(
        'className prop when given a badly cased alias',
        async render => {
          const e = await render(<div cLASs="test" />, 1);
          expect(e.className).toBe('test');
        },
      );
      itRenders('class for custom elements', async render => {
        const e = await render(<div is="custom-element" class="test" />, 0);
        expect(e.getAttribute('class')).toBe('test');
      });
      itRenders('className for custom elements', async render => {
        const e = await render(<div is="custom-element" className="test" />, 0);
        expect(e.getAttribute('className')).toBe('test');