**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('a div with multiple children', async render => {
        const e = await render(
          <div id="parent"><div id="child1" /><div id="child2" /></div>,
        );
        expect(e.id).toBe('parent');
        expect(e.childNodes.length).toBe(2);
        expect(e.childNodes[0].id).toBe('child1');
        expect(e.childNodes[0].childNodes.length).toBe(0);
        expect(e.childNodes[1].id).toBe('child2');
        expect(e.childNodes[1].childNodes.length).toBe(0);
      });
      itRenders(
        'a div with multiple children separated by whitespace',
        async render => {
          const e = await render(
            <div id="parent"><div id="child1" /> <div id="child2" /></div>,
          );
          expect(e.id).toBe('parent');
          let child1, child2, textNode;
          if (ReactDOMFeatureFlags.useFiber) {
            
            
            expect(e.childNodes.length).toBe(3);
            child1 = e.childNodes[0];
            textNode = e.childNodes[1];
            child2 = e.childNodes[2];
          } else {