**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: Instance | TextInstance,
  ): null | Instance | TextInstance {
    let node = instance.nextSibling;
    
    while (
      node &&
      node.nodeType !== ELEMENT_NODE &&
      node.nodeType !== TEXT_NODE
    ) {
      node = node.nextSibling;
    }
    return (node: any);
  },
  getFirstHydratableChild(
    parentInstance: Container | Instance,
  ): null | Instance | TextInstance {
    let next = parentInstance.firstChild;
    
    while (
      next &&
      next.nodeType !== ELEMENT_NODE &&
      next.nodeType !== TEXT_NODE
    ) {
      next = next.nextSibling;
    }
    return (next: any);
  },
  hydrateInstance(
    instance: Instance,
    type: string,