**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.
props: Props,
    rootContainerInstance: Container,
    internalInstanceHandle: Object,
  ): null | Array<mixed> {
    precacheFiberNode(internalInstanceHandle, instance);
    
    
    updateFiberProps(instance, props);
    return diffHydratedProperties(instance, type, props, rootContainerInstance);
  },
  hydrateTextInstance(
    textInstance: TextInstance,
    text: string,
    internalInstanceHandle: Object,
  ): boolean {
    precacheFiberNode(internalInstanceHandle, textInstance);
    return diffHydratedText(textInstance, text);
  },
  didNotHydrateInstance(
    parentInstance: Instance | Container,
    instance: Instance | TextInstance,
  ) {
    if (instance.nodeType === 1) {
      warnForDeletedHydratableElement(parentInstance, (instance: any));
    } else {
      warnForDeletedHydratableText(parentInstance, (instance: any));
    }
  },
  didNotFindHydratableInstance(
    parentInstance: Instance | Container,