**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 parent = e.parentNode;
        expect(parent.childNodes[0].tagName).toBe('DIV');
        expect(parent.childNodes[1].tagName).toBe('SPAN');
        expect(parent.childNodes[2].tagName).toBe('P');
        expect(parent.childNodes[3].tagName).toBe('H2');
        expect(parent.childNodes[4].tagName).toBe('H3');
      });
      itRenders('a nested array', async render => {
        let e = await render([
          [<div key={1}>text1</div>],
          <span key={1}>text2</span>,
          [[[null, <p key={1} />], false]],
        ]);
        let parent = e.parentNode;
        expect(parent.childNodes[0].tagName).toBe('DIV');
        expect(parent.childNodes[1].tagName).toBe('SPAN');
        expect(parent.childNodes[2].tagName).toBe('P');
      });
      itRenders('an iterable', async render => {
        const threeDivIterable = {
          '@@iterator': function() {
            var i = 0;
            return {
              next: function() {
                if (i++ < 3) {
                  return {value: <div key={i} />, done: false};
                } else {
                  return {value: undefined, done: true};
                }