**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('no unknown events', async render => {
      const e = await render(
        <div onunknownevent="alert(&quot;hack&quot;)" />,
        1,
      );
      expect(e.getAttribute('onunknownevent')).toBe(null);
    });
  });
  describe('elements and children', function() {
    function expectNode(node, type, value) {
      expect(node).not.toBe(null);
      expect(node.nodeType).toBe(type);
      expect(node.nodeValue).toMatch(value);
    }
    function expectTextNode(node, text) {
      if (ReactDOMFeatureFlags.useFiber) {
        
        expectNode(node, TEXT_NODE_TYPE, text);
      } else {
        
        
        expectNode(node, COMMENT_NODE_TYPE, / react-text: [0-9]+ /);
        if (text.length > 0) {
          node = node.nextSibling;
          expectNode(node, TEXT_NODE_TYPE, text);
        }
        expectNode(node.nextSibling, COMMENT_NODE_TYPE, / \/react-text /);
      }
    }
    function expectEmptyNode(node) {