**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.
});
        it('should not blow away user-entered text on successful reconnect to an uncontrolled checkbox', () =>
          testUserInteractionBeforeClientRender(
            <input type="checkbox" defaultChecked={true} />,
            true,
            false,
            'checked',
          ));
        it('should not blow away user-entered text on successful reconnect to a controlled checkbox', async () => {
          let changeCount = 0;
          await testUserInteractionBeforeClientRender(
            <ControlledCheckbox onChange={() => changeCount++} />,
            true,
            false,
            'checked',
          );
          expect(changeCount).toBe(0);
        });
        
        
        xit(
          'should not blow away user-entered text on successful reconnect to an uncontrolled textarea',
          () =>
            testUserInteractionBeforeClientRender(
              <textarea defaultValue="Hello" />,
            ),
        );
        
        
        xit(