**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.
});
    });
    describe('htmlFor property', function() {
      itRenders('htmlFor with string value', async render => {
        const e = await render(<div htmlFor="myFor" />);
        expect(e.getAttribute('for')).toBe('myFor');
      });
      itRenders('no badly cased htmlfor', async render => {
        const e = await render(<div htmlfor="myFor" />, 1);
        expect(e.hasAttribute('for')).toBe(false);
        expect(e.getAttribute('htmlfor')).toBe('myFor');
      });
      itRenders('htmlFor with an empty string', async render => {
        const e = await render(<div htmlFor="" />);
        expect(e.getAttribute('for')).toBe('');
      });
      itRenders('no htmlFor prop with true value', async render => {
        const e = await render(<div htmlFor={true} />, 1);
        expect(e.hasAttribute('for')).toBe(false);
      });
      itRenders('no htmlFor prop with false value', async render => {
        const e = await render(<div htmlFor={false} />, 1);
        expect(e.hasAttribute('for')).toBe(false);
      });
      itRenders('no htmlFor prop with null value', async render => {
        const e = await render(<div htmlFor={null} />);
        expect(e.hasAttribute('htmlFor')).toBe(false);
      });
      itRenders('htmlFor attribute on custom elements', async render => {
        const e = await render(<div is="custom-element" htmlFor="test" />);