**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.
validateProperties: validateInputProperties,
  } = ReactDOMNullInputValuePropHook;
  var {
    validateProperties: validateUnknownProperties,
  } = ReactDOMUnknownPropertyHook;
}
var didWarnInvalidHydration = false;
var didWarnShadyDOM = false;
var listenTo = ReactBrowserEventEmitter.listenTo;
var registrationNameModules = EventPluginRegistry.registrationNameModules;
var DANGEROUSLY_SET_INNER_HTML = 'dangerouslySetInnerHTML';
var SUPPRESS_CONTENT_EDITABLE_WARNING = 'suppressContentEditableWarning';
var CHILDREN = 'children';
var STYLE = 'style';
var HTML = '__html';
var {Namespaces: {html: HTML_NAMESPACE}, getIntrinsicNamespace} = DOMNamespaces;
if (__DEV__) {
  var warnedUnknownTags = {
    
    
    
    
    time: true,
  };
  var validatePropertiesInDevelopment = function(type, props) {
    validateARIAProperties(type, props);
    validateInputProperties(type, props);
    validateUnknownProperties(type, props);
  };
  var warnForTextDifference = function(serverText: string, clientText: string) {