**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.
ReactDOMFiberSelect.postUpdateWrapper(domElement, nextRawProps);
        break;
    }
  },
  diffHydratedProperties(
    domElement: Element,
    tag: string,
    rawProps: Object,
    rootContainerElement: Element | Document,
  ): null | Array<mixed> {
    if (__DEV__) {
      var isCustomComponentTag = isCustomComponent(tag, rawProps);
      validatePropertiesInDevelopment(tag, rawProps);
      if (isCustomComponentTag && !didWarnShadyDOM && domElement.shadyRoot) {
        warning(
          false,
          '%s is using shady DOM. Using shady DOM with React can ' +
            'cause things to break subtly.',
          getCurrentFiberOwnerName() || 'A component',
        );
        didWarnShadyDOM = true;
      }
    }
    
    switch (tag) {
      case 'iframe':
      case 'object':
        ReactBrowserEventEmitter.trapBubbledEvent(