**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 (
    !foundDevTools &&
    ExecutionEnvironment.canUseDOM &&
    window.top === window.self
  ) {
    
    if (
      (navigator.userAgent.indexOf('Chrome') > -1 &&
        navigator.userAgent.indexOf('Edge') === -1) ||
      navigator.userAgent.indexOf('Firefox') > -1
    ) {
      const protocol = window.location.protocol;
      
      if (/^(https?|file):$/.test(protocol)) {
        console.info(
          '%cDownload the React DevTools ' +
            'for a better development experience: ' +
            'https:
            (protocol === 'file:'
              ? '\nYou might need to use a local HTTP server (instead of file:
                  'https:
              : ''),
          'font-weight:bold',
        );
      }
    }
  }
}
module.exports = ReactDOMFiber;