**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.
expect(e.childNodes.length).toBe(5);
            child1 = e.childNodes[0];
            textNode = e.childNodes[1];
            child2 = e.childNodes[4];
          }
          expect(child1.id).toBe('child1');
          expect(child1.childNodes.length).toBe(0);
          expectTextNode(textNode, ' ');
          expect(child2.id).toBe('child2');
          expect(child2.childNodes.length).toBe(0);
        },
      );
      itRenders(
        'a div with a single child surrounded by whitespace',
        async render => {
          
          const e = await render(<div id="parent">  <div id="child" />   </div>); 
          let textNode1, child, textNode2;
          if (ReactDOMFeatureFlags.useFiber) {
            
            
            expect(e.childNodes.length).toBe(3);
            textNode1 = e.childNodes[0];
            child = e.childNodes[1];
            textNode2 = e.childNodes[2];
          } else {
            
            
            
            expect(e.childNodes.length).toBe(7);