**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.
if (__DEV__) {
  var lowPriorityWarning = require('lowPriorityWarning');
  var warning = require('fbjs/lib/warning');
  var validateDOMNesting = require('validateDOMNesting');
  var {updatedAncestorInfo} = validateDOMNesting;
  if (
    typeof Map !== 'function' ||
    Map.prototype == null ||
    typeof Map.prototype.forEach !== 'function' ||
    typeof Set !== 'function' ||
    Set.prototype == null ||
    typeof Set.prototype.clear !== 'function' ||
    typeof Set.prototype.forEach !== 'function'
  ) {
    warning(
      false,
      'React depends on Map and Set built-in types. Make sure that you load a ' +
        'polyfill in older browsers. http://fb.me/react-polyfills',
    );
  }
}
require('ReactDOMClientInjection');
require('ReactDOMInjection');
ReactControlledComponent.injection.injectFiberControlledHostComponent(
  ReactDOMFiberComponent,
);
findDOMNode._injectFiber(function(fiber: Fiber) {
  return DOMRenderer.findHostInstance(fiber);
});
type DOMContainer =