**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.
it('should error reconnecting added style values', () =>
        expectMarkupMismatch(
          <div style={{}} />,
          <div style={{width: '1px'}} />,
        ));
      it('should error reconnecting different style values', () =>
        expectMarkupMismatch(
          <div style={{width: '1px'}} />,
          <div style={{width: '2px'}} />,
        ));
      it('should reconnect number and string versions of a number', () =>
        expectMarkupMatch(
          <div style={{width: '1px', height: 2}} />,
          <div style={{width: 1, height: '2px'}} />,
        ));
      it('should error reconnecting reordered style values', () =>
        expectMarkupMismatch(
          <div style={{width: '1px', fontSize: '2px'}} />,
          <div style={{fontSize: '2px', width: '1px'}} />,
        ));
    });
    describe('text nodes', function() {
      it('should error reconnecting different text', () =>
        expectMarkupMismatch(<div>Text</div>, <div>Other Text</div>));
      it('should reconnect a div with a number and string version of number', () =>
        expectMarkupMatch(<div>{2}</div>, <div>2</div>));
      it('should error reconnecting different numbers', () =>
        expectMarkupMismatch(<div>{2}</div>, <div>{3}</div>));
      it('should error reconnecting different number from text', () =>
        expectMarkupMismatch(<div>{2}</div>, <div>3</div>));