**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.
return false;
    }
  },
  findDOMNode: findDOMNode,
  
  
  unstable_createPortal: createPortal,
  unstable_batchedUpdates: ReactGenericBatching.batchedUpdates,
  unstable_deferredUpdates: DOMRenderer.deferredUpdates,
  flushSync: DOMRenderer.flushSync,
  __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED: {
    
    EventPluginHub: require('EventPluginHub'),
    
    EventPluginRegistry: require('EventPluginRegistry'),
    EventPropagators: require('EventPropagators'),
    ReactControlledComponent,
    ReactDOMComponentTree,
    ReactDOMEventListener: require('ReactDOMEventListener'),
  },
};
const foundDevTools = injectInternals({
  findFiberByHostInstance: ReactDOMComponentTree.getClosestInstanceFromNode,
  findHostInstanceByFiber: DOMRenderer.findHostInstance,
  
  bundleType: __DEV__ ? 1 : 0,
  version: ReactVersion,
  rendererPackageName: 'react-dom',
});
if (__DEV__) {