**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.
const ancestorInfo = updatedAncestorInfo(
        parentHostContextDev.ancestorInfo,
        type,
        null,
      );
      return {namespace, ancestorInfo};
    }
    const parentNamespace = ((parentHostContext: any): HostContextProd);
    return getChildNamespace(parentNamespace, type);
  },
  getPublicInstance(instance) {
    return instance;
  },
  prepareForCommit(): void {
    eventsEnabled = ReactBrowserEventEmitter.isEnabled();
    selectionInformation = ReactInputSelection.getSelectionInformation();
    ReactBrowserEventEmitter.setEnabled(false);
  },
  resetAfterCommit(): void {
    ReactInputSelection.restoreSelection(selectionInformation);
    selectionInformation = null;
    ReactBrowserEventEmitter.setEnabled(eventsEnabled);
    eventsEnabled = null;
  },
  createInstance(
    type: string,
    props: Props,
    rootContainerInstance: Container,
    hostContext: HostContext,
    internalInstanceHandle: Object,