**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(parent.childNodes.length).toBe(3);
            expectTextNode(parent.childNodes[0], 'a');
            expectTextNode(parent.childNodes[1], 'b');
            expectTextNode(parent.childNodes[2], 'c');
          }
        });
      }
    });
    describe('escaping >, <, and &', function() {
      itRenders('>,<, and & as single child', async render => {
        const e = await render(<div>{'<span>Text&quot;</span>'}</div>);
        expect(e.childNodes.length).toBe(1);
        expectNode(e.firstChild, TEXT_NODE_TYPE, '<span>Text&quot;</span>');
      });
      itRenders('>,<, and & as multiple children', async render => {
        const e = await render(
          <div>{'<span>Text1&quot;</span>'}{'<span>Text2&quot;</span>'}</div>,
        );
        if (ReactDOMFeatureFlags.useFiber) {
          
          if (
            render === serverRender ||
            render === clientRenderOnServerString ||
            render === streamRender
          ) {
            expect(e.childNodes.length).toBe(3);
            expectTextNode(e.childNodes[0], '<span>Text1&quot;</span>');
            expectTextNode(e.childNodes[2], '<span>Text2&quot;</span>');
          } else {
            expect(e.childNodes.length).toBe(2);