**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.
textNode1 = e.childNodes[0];
            child = e.childNodes[3];
            textNode2 = e.childNodes[4];
          }
          expect(e.id).toBe('parent');
          expectTextNode(textNode1, '  ');
          expect(child.id).toBe('child');
          expect(child.childNodes.length).toBe(0);
          expectTextNode(textNode2, '   ');
        },
      );
      if (ReactDOMFeatureFlags.useFiber) {
        itRenders('a composite with multiple children', async render => {
          const Component = props => props.children;
          const e = await render(
            <Component>{['a', 'b', [undefined], [[false, 'c']]]}</Component>,
          );
          let parent = e.parentNode;
          if (
            render === serverRender ||
            render === clientRenderOnServerString ||
            render === streamRender
          ) {
            
            
            expect(parent.childNodes.length).toBe(5);
            expectTextNode(parent.childNodes[0], 'a');
            expectTextNode(parent.childNodes[2], 'b');
            expectTextNode(parent.childNodes[4], 'c');
          } else {