**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.
<div id="parent"><div id="child1" /><div id="child2" /></div>,
        ));
      it('should error reconnecting a div with children separated by different whitespace', () =>
        expectMarkupMismatch(
          <div id="parent"><div id="child1" /> <div id="child2" /></div>,
          
          <div id="parent"><div id="child1" />      <div id="child2" /></div>, 
        ));
      it('can distinguish an empty component from a dom node', () =>
        expectMarkupMismatch(
          <div><span /></div>,
          <div><EmptyComponent /></div>,
        ));
      it('can distinguish an empty component from an empty text component', () =>
        (ReactDOMFeatureFlags.useFiber
          ? expectMarkupMatch
          : expectMarkupMismatch)(
          <div><EmptyComponent /></div>,
          <div>{''}</div>,
        ));
    });
    
    it('should error reconnecting a div with different dangerouslySetInnerHTML', () =>
      expectMarkupMismatch(
        <div dangerouslySetInnerHTML={{__html: "<span id='child1'/>"}} />,
        <div dangerouslySetInnerHTML={{__html: "<span id='child2'/>"}} />,
      ));
  });
});