**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.
ExecutionEnvironment = require('ExecutionEnvironment');
  PropTypes = require('prop-types');
  React = require('react');
  ReactDOM = require('react-dom');
  ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
  ReactTestUtils = require('react-dom/test-utils');
  
  
  
  jest.resetModuleRegistry();
  ReactDOMServer = require('react-dom/server');
}
describe('ReactDOMServerIntegration', () => {
  beforeEach(() => {
    resetModules();
    ExecutionEnvironment.canUseDOM = false;
  });
  describe('basic rendering', function() {
    itRenders('a blank div', async render => {
      const e = await render(<div />);
      expect(e.tagName).toBe('DIV');
    });
    itRenders('a self-closing tag', async render => {
      const e = await render(<br />);
      expect(e.tagName).toBe('BR');
    });
    itRenders('a self-closing tag as a child', async render => {
      const e = await render(<div><br /></div>);
      expect(e.childNodes.length).toBe(1);