**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.
element.type.prototype != null &&
      (element.type.prototype: any).unstable_isAsyncReactComponent === true;
    const priorityLevel = getPriorityContext(current, forceAsync);
    const nextState = {element};
    callback = callback === undefined ? null : callback;
    if (__DEV__) {
      warning(
        callback === null || typeof callback === 'function',
        'render(...): Expected the last optional `callback` argument to be a ' +
          'function. Instead received: %s.',
        callback,
      );
    }
    addTopLevelUpdate(current, nextState, callback, priorityLevel);
    scheduleUpdate(current, priorityLevel);
  }
  return {
    createContainer(containerInfo: C): OpaqueRoot {
      return createFiberRoot(containerInfo);
    },
    updateContainer(
      element: ReactNodeList,
      container: OpaqueRoot,
      parentComponent: ?React$Component<any, any>,
      callback: ?Function,
    ): void {
      
      const current = container.current;
      if (__DEV__) {
        if (ReactFiberInstrumentation.debugTool) {