**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.
const clientRenderOnBadMarkup = async (element, errorCount = 0) => {
  
  var domElement = document.createElement('div');
  domElement.innerHTML =
    '<div id="badIdWhichWillCauseMismatch" data-reactroot="" data-reactid="1"></div>';
  await renderIntoDom(element, domElement, true, errorCount + 1);
  
  var hydratedTextContent = domElement.textContent;
  
  const cleanDomElement = document.createElement('div');
  ExecutionEnvironment.canUseDOM = true;
  await asyncReactDOMRender(element, cleanDomElement, true);
  ExecutionEnvironment.canUseDOM = false;
  
  const cleanTextContent = cleanDomElement.textContent;
  
  expect(hydratedTextContent).toBe(cleanTextContent);
  
  throw new BadMarkupExpected();
};
function itRenders(desc, testFn) {
  it(`renders ${desc} with server string render`, () => testFn(serverRender));
  if (ReactDOMFeatureFlags.useFiber) {
    it(`renders ${desc} with server stream render`, () => testFn(streamRender));
  }
  itClientRenders(desc, testFn);
}
function itClientRenders(desc, testFn) {