**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.
DOMRenderer.unbatchedUpdates(() => {
        renderSubtreeIntoContainer(null, null, container, false, () => {
          container._reactRootContainer = null;
        });
      });
      
      
      return true;
    } else {
      if (__DEV__) {
        const rootEl = getReactRootElementInContainer(container);
        const hasNonRootReactChild = !!(rootEl &&
          ReactDOMComponentTree.getInstanceFromNode(rootEl));
        
        const isContainerReactRoot =
          container.nodeType === 1 &&
          isValidContainer(container.parentNode) &&
          !!container.parentNode._reactRootContainer;
        warning(
          !hasNonRootReactChild,
          "unmountComponentAtNode(): The node you're attempting to unmount " +
            'was rendered by React and is not a top-level container. %s',
          isContainerReactRoot
            ? 'You may have accidentally passed in a React root node instead ' +
                'of its container.'
            : 'Instead, have the parent component update its state and ' +
                'rerender in order to remove this component.',
        );
      }