**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.style.width).toBe('30px');
      });
      itRenders('relevant styles with px', async render => {
        const e = await render(
          <div
            style={{
              left: 0,
              margin: 16,
              opacity: 0.5,
              padding: '4px',
            }}
          />,
        );
        expect(e.style.left).toBe('0px');
        expect(e.style.margin).toBe('16px');
        expect(e.style.opacity).toBe('0.5');
        expect(e.style.padding).toBe('4px');
      });
      itRenders('custom properties', async render => {
        const e = await render(<div style={{'--foo': 5}} />);
        
        
        expect(e.style.Foo).toBe('5');
      });
      itRenders('no undefined styles', async render => {
        const e = await render(
          <div style={{color: undefined, width: '30px'}} />,
        );
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('30px');