**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('an img', async render => {
      const e = await render(<img />);
      expect(e.childNodes.length).toBe(0);
      expect(e.nextSibling).toBe(null);
      expect(e.tagName).toBe('IMG');
    });
    itRenders('a button', async render => {
      const e = await render(<button />);
      expect(e.childNodes.length).toBe(0);
      expect(e.nextSibling).toBe(null);
      expect(e.tagName).toBe('BUTTON');
    });
    itRenders('a div with dangerouslySetInnerHTML', async render => {
      const e = await render(
        <div dangerouslySetInnerHTML={{__html: "<span id='child'/>"}} />,
      );
      expect(e.childNodes.length).toBe(1);
      expect(e.firstChild.tagName).toBe('SPAN');
      expect(e.firstChild.getAttribute('id')).toBe('child');
      expect(e.firstChild.childNodes.length).toBe(0);
    });
    describe('newline-eating elements', function() {
      itRenders(
        'a newline-eating tag with content not starting with \\n',
        async render => {
          const e = await render(<pre>Hello</pre>);
          expect(e.textContent).toBe('Hello');
        },
      );
      itRenders(