**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.
callback: ?Function,
  ) {
    invariant(
      parentComponent != null && ReactInstanceMap.has(parentComponent),
      'parentComponent must be a valid React Component',
    );
    return renderSubtreeIntoContainer(
      parentComponent,
      element,
      containerNode,
      false,
      callback,
    );
  },
  unmountComponentAtNode(container: DOMContainer) {
    invariant(
      isValidContainer(container),
      'unmountComponentAtNode(...): Target container is not a DOM element.',
    );
    if (container._reactRootContainer) {
      if (__DEV__) {
        const rootEl = getReactRootElementInContainer(container);
        const renderedByDifferentReact =
          rootEl && !ReactDOMComponentTree.getInstanceFromNode(rootEl);
        warning(
          !renderedByDifferentReact,
          "unmountComponentAtNode(): The node you're attempting to unmount " +
            'was rendered by another copy of React.',
        );
      }