**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.
container.removeChild(rootSibling);
      }
    }
    if (__DEV__) {
      if (shouldHydrate && !forceHydrate && !warnedAboutHydrateAPI) {
        warnedAboutHydrateAPI = true;
        lowPriorityWarning(
          false,
          'render(): Calling ReactDOM.render() to hydrate server-rendered markup ' +
            'will stop working in React v17. Replace the ReactDOM.render() call ' +
            'with ReactDOM.hydrate() if you want React to attach to the server HTML.',
        );
      }
    }
    const newRoot = DOMRenderer.createContainer(container);
    root = container._reactRootContainer = newRoot;
    
    DOMRenderer.unbatchedUpdates(() => {
      DOMRenderer.updateContainer(children, newRoot, parentComponent, callback);
    });
  } else {
    DOMRenderer.updateContainer(children, root, parentComponent, callback);
  }
  return DOMRenderer.getPublicRootInstance(root);
}
function createPortal(
  children: ReactNodeList,
  container: DOMContainer,
  key: ?string = null,
) {