**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.
topLoadedData: 'loadeddata',
  topLoadedMetadata: 'loadedmetadata',
  topLoadStart: 'loadstart',
  topPause: 'pause',
  topPlay: 'play',
  topPlaying: 'playing',
  topProgress: 'progress',
  topRateChange: 'ratechange',
  topSeeked: 'seeked',
  topSeeking: 'seeking',
  topStalled: 'stalled',
  topSuspend: 'suspend',
  topTimeUpdate: 'timeupdate',
  topVolumeChange: 'volumechange',
  topWaiting: 'waiting',
};
function trapClickOnNonInteractiveElement(node: HTMLElement) {
  
  
  
  
  
  
  
  
  
  node.onclick = emptyFunction;
}
function setInitialDOMProperties(
  domElement: Element,