**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.
invariant(
    isValidContainer(container),
    'Target container is not a DOM element.',
  );
  
  return ReactPortal.createPortal(children, container, null, key);
}
var ReactDOMFiber = {
  createPortal,
  hydrate(element: React$Node, container: DOMContainer, callback: ?Function) {
    
    return renderSubtreeIntoContainer(null, element, container, true, callback);
  },
  render(
    element: React$Element<any>,
    container: DOMContainer,
    callback: ?Function,
  ) {
    return renderSubtreeIntoContainer(
      null,
      element,
      container,
      false,
      callback,
    );
  },
  unstable_renderSubtreeIntoContainer(
    parentComponent: React$Component<any, any>,
    element: React$Element<any>,
    containerNode: DOMContainer,