**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.
return renderIntoDom(element, div, false, errorCount);
};
const clientRenderOnServerString = async (element, errorCount = 0) => {
  const markup = await renderIntoString(element, errorCount);
  resetModules();
  var domElement = document.createElement('div');
  domElement.innerHTML = markup;
  let serverNode = domElement.firstChild;
  const firstClientNode = await renderIntoDom(
    element,
    domElement,
    true,
    errorCount,
  );
  let clientNode = firstClientNode;
  
  while (serverNode || clientNode) {
    expect(serverNode != null).toBe(true);
    expect(clientNode != null).toBe(true);
    expect(clientNode.nodeType).toBe(serverNode.nodeType);
    
    
    
    expect(serverNode === clientNode).toBe(true);
    serverNode = serverNode.nextSibling;
    clientNode = clientNode.nextSibling;
  }
  return firstClientNode;
};
function BadMarkupExpected() {}