**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.
},
  commitUpdate(
    domElement: Instance,
    updatePayload: Array<mixed>,
    type: string,
    oldProps: Props,
    newProps: Props,
    internalInstanceHandle: Object,
  ): void {
    
    
    updateFiberProps(domElement, newProps);
    
    updateProperties(domElement, updatePayload, type, oldProps, newProps);
  },
  shouldSetTextContent(type: string, props: Props): boolean {
    return (
      type === 'textarea' ||
      typeof props.children === 'string' ||
      typeof props.children === 'number' ||
      (typeof props.dangerouslySetInnerHTML === 'object' &&
        props.dangerouslySetInnerHTML !== null &&
        typeof props.dangerouslySetInnerHTML.__html === 'string')
    );
  },
  resetTextContent(domElement: Instance): void {
    domElement.textContent = '';
  },
  shouldDeprioritizeSubtree(type: string, props: Props): boolean {
    return !!props.hidden;