**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.
if (container._reactRootContainer && container.nodeType !== COMMENT_NODE) {
      const hostInstance = DOMRenderer.findHostInstanceWithNoPortals(
        container._reactRootContainer.current,
      );
      if (hostInstance) {
        warning(
          hostInstance.parentNode === container,
          'render(...): It looks like the React-rendered content of this ' +
            'container was removed without using React. This is not ' +
            'supported and will cause errors. Instead, call ' +
            'ReactDOM.unmountComponentAtNode to empty a container.',
        );
      }
    }
    const isRootRenderedBySomeReact = !!container._reactRootContainer;
    const rootEl = getReactRootElementInContainer(container);
    const hasNonRootReactChild = !!(rootEl &&
      ReactDOMComponentTree.getInstanceFromNode(rootEl));
    warning(
      !hasNonRootReactChild || isRootRenderedBySomeReact,
      'render(...): Replacing React-rendered children with a new root ' +
        'component. If you intended to update the children of this node, ' +
        'you should instead have the existing children update their state ' +
        'and render the new components instead of calling ReactDOM.render.',
    );
    warning(
      container.nodeType !== ELEMENT_NODE ||
        !((container: any): Element).tagName ||
        ((container: any): Element).tagName.toUpperCase() !== 'BODY',
      'render(): Rendering components directly into document.body is ' +