**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.
},
  createTextInstance(
    text: string,
    rootContainerInstance: Container,
    hostContext: HostContext,
    internalInstanceHandle: Object,
  ): TextInstance {
    if (__DEV__) {
      const hostContextDev = ((hostContext: any): HostContextDev);
      validateDOMNesting(null, text, null, hostContextDev.ancestorInfo);
    }
    var textNode: TextInstance = document.createTextNode(text);
    precacheFiberNode(internalInstanceHandle, textNode);
    return textNode;
  },
  commitTextUpdate(
    textInstance: TextInstance,
    oldText: string,
    newText: string,
  ): void {
    textInstance.nodeValue = newText;
  },
  appendChild(parentInstance: Instance, child: Instance | TextInstance): void {
    parentInstance.appendChild(child);
  },
  appendChildToContainer(
    container: Container,
    child: Instance | TextInstance,
  ): void {
    if (container.nodeType === COMMENT_NODE) {