**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.
var ReactFiberReconciler = require('ReactFiberReconciler');
var ReactInputSelection = require('ReactInputSelection');
var ReactInstanceMap = require('ReactInstanceMap');
var ReactPortal = require('ReactPortal');
var ReactVersion = require('ReactVersion');
var {injectInternals} = require('ReactFiberDevToolsHook');
var {
  ELEMENT_NODE,
  TEXT_NODE,
  COMMENT_NODE,
  DOCUMENT_NODE,
  DOCUMENT_FRAGMENT_NODE,
} = require('HTMLNodeType');
var {ROOT_ATTRIBUTE_NAME} = require('DOMProperty');
var findDOMNode = require('findDOMNode');
var invariant = require('fbjs/lib/invariant');
var {getChildNamespace} = DOMNamespaces;
var {
  createElement,
  setInitialProperties,
  diffProperties,
  updateProperties,
  diffHydratedProperties,
  diffHydratedText,
  warnForDeletedHydratableElement,
  warnForDeletedHydratableText,
  warnForInsertedHydratedElement,
  warnForInsertedHydratedText,
} = ReactDOMFiberComponent;
var {precacheFiberNode, updateFiberProps} = ReactDOMComponentTree;