**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.
expect(e.firstChild.tagName).toBe('BR');
    });
    if (ReactDOMFeatureFlags.useFiber) {
      itRenders('a string', async render => {
        let e = await render('Hello');
        expect(e.nodeType).toBe(3);
        expect(e.nodeValue).toMatch('Hello');
      });
      itRenders('a number', async render => {
        let e = await render(42);
        expect(e.nodeType).toBe(3);
        expect(e.nodeValue).toMatch('42');
      });
      itRenders('an array with one child', async render => {
        let e = await render([<div key={1}>text1</div>]);
        let parent = e.parentNode;
        expect(parent.childNodes[0].tagName).toBe('DIV');
      });
      itRenders('an array with several children', async render => {
        let Header = props => {
          return <p>header</p>;
        };
        let Footer = props => {
          return [<h2 key={1}>footer</h2>, <h3 key={2}>about</h3>];
        };
        let e = await render([
          <div key={1}>text1</div>,
          <span key={2}>text2</span>,
          <Header key={3} />,
          <Footer key={4} />,