**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.
* @providesModule ReactDOMFiberComponent
 * @flow
 */
'use strict';
var CSSPropertyOperations = require('CSSPropertyOperations');
var DOMNamespaces = require('DOMNamespaces');
var DOMProperty = require('DOMProperty');
var DOMPropertyOperations = require('DOMPropertyOperations');
var EventPluginRegistry = require('EventPluginRegistry');
var ReactBrowserEventEmitter = require('ReactBrowserEventEmitter');
var ReactDOMFiberInput = require('ReactDOMFiberInput');
var ReactDOMFiberOption = require('ReactDOMFiberOption');
var ReactDOMFiberSelect = require('ReactDOMFiberSelect');
var ReactDOMFiberTextarea = require('ReactDOMFiberTextarea');
var {getCurrentFiberOwnerName} = require('ReactDebugCurrentFiber');
var {DOCUMENT_NODE, DOCUMENT_FRAGMENT_NODE} = require('HTMLNodeType');
var assertValidProps = require('assertValidProps');
var emptyFunction = require('fbjs/lib/emptyFunction');
var inputValueTracking = require('inputValueTracking');
var isCustomComponent = require('isCustomComponent');
var setInnerHTML = require('setInnerHTML');
var setTextContent = require('setTextContent');
if (__DEV__) {
  var warning = require('fbjs/lib/warning');
  var {getCurrentFiberStackAddendum} = require('ReactDebugCurrentFiber');
  var ReactDOMInvalidARIAHook = require('ReactDOMInvalidARIAHook');
  var ReactDOMNullInputValuePropHook = require('ReactDOMNullInputValuePropHook');
  var ReactDOMUnknownPropertyHook = require('ReactDOMUnknownPropertyHook');
  var {validateProperties: validateARIAProperties} = ReactDOMInvalidARIAHook;
  var {