**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.
render() {
          return <div ref="myDiv" />;
        }
      }
      const markup = ReactDOMServer.renderToString(<RefsComponent />);
      const root = document.createElement('div');
      root.innerHTML = markup;
      let component = null;
      resetModules();
      await asyncReactDOMRender(
        <RefsComponent ref={e => (component = e)} />,
        root,
        true,
      );
      expect(component.refs.myDiv).toBe(root.firstChild);
    });
  });
  describe('reconnecting to server markup', function() {
    var EmptyComponent;
    beforeEach(() => {
      EmptyComponent = class extends React.Component {
        render() {