**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('an input value overriding defaultValue', async render => {
        const e = await render(
          <input value="foo" defaultValue="bar" readOnly={true} />,
          1,
        );
        expect(e.value).toBe('foo');
        expect(e.getAttribute('value')).toBe('foo');
        expect(e.getAttribute('defaultValue')).toBe(null);
      });
      itRenders(
        'an input value overriding defaultValue no matter the prop order',
        async render => {
          const e = await render(
            <input defaultValue="bar" value="foo" readOnly={true} />,
            1,
          );
          expect(e.value).toBe('foo');
          expect(e.getAttribute('value')).toBe('foo');
          expect(e.getAttribute('defaultValue')).toBe(null);
        },
      );
    });
    describe('checkboxes', function() {
      itRenders('a checkbox that is checked with an onChange', async render => {
        const e = await render(
          <input type="checkbox" checked={true} onChange={() => {}} />,
        );
        expect(e.checked).toBe(true);
      });
      itRenders('a checkbox that is checked with readOnly', async render => {