**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.
): Instance {
    let parentNamespace: string;
    if (__DEV__) {
      
      const hostContextDev = ((hostContext: any): HostContextDev);
      validateDOMNesting(type, null, null, hostContextDev.ancestorInfo);
      if (
        typeof props.children === 'string' ||
        typeof props.children === 'number'
      ) {
        const string = '' + props.children;
        const ownAncestorInfo = updatedAncestorInfo(
          hostContextDev.ancestorInfo,
          type,
          null,
        );
        validateDOMNesting(null, string, null, ownAncestorInfo);
      }
      parentNamespace = hostContextDev.namespace;
    } else {
      parentNamespace = ((hostContext: any): HostContextProd);
    }
    const domElement: Instance = createElement(
      type,
      props,
      rootContainerInstance,
      parentNamespace,
    );
    precacheFiberNode(internalInstanceHandle, domElement);
    updateFiberProps(domElement, props);