**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(`renders ${desc} with clean client render`, () =>
    testFn(clientCleanRender));
  it(`renders ${desc} with client render on top of good server markup`, () =>
    testFn(clientRenderOnServerString));
  it(`renders ${desc} with client render on top of bad server markup`, async () => {
    try {
      await testFn(clientRenderOnBadMarkup);
    } catch (x) {
      
      if (!(x instanceof BadMarkupExpected)) {
        
        throw x;
      }
    }
  });
}
function itThrows(desc, testFn, partialMessage) {
  it(`throws ${desc}`, () => {
    return testFn().then(
      () => expect(false).toBe('The promise resolved and should not have.'),
      err => {
        expect(err).toBeInstanceOf(Error);
        expect(err.message).toContain(partialMessage);
      },
    );
  });
}
function itThrowsWhenRendering(desc, testFn, partialMessage) {