**Do you want to request a *feature* or report a *bug*?**
Bug

**What is the current behavior?**
I've started recieving this warning about my SVGs which is using [feGaussianBlur](https://developer.mozilla.org/en-US/docs/Web/SVG/Element/feGaussianBlur): 
`Warning: Expected server HTML to contain a matching <feGaussianBlur> in <filter>.`

**What is the expected behavior?**
Should be no warnings because there are no difference.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Such behavior started right after updating to `React 16.0.0-rc.3` and never happened with `React 15.6.1`
itRenders('cased custom attributes', async render => {
        const e = await render(<div fooBar="test" />, 1);
        expect(e.getAttribute('foobar')).toBe('test');
      });
    });
    itRenders('no HTML events', async render => {
      const e = await render(<div onClick={() => {}} />);
      expect(e.getAttribute('onClick')).toBe(null);
      expect(e.getAttribute('onClick')).toBe(null);
      expect(e.getAttribute('click')).toBe(null);
    });
    itRenders('no unknown events', async render => {
      const e = await render(
        <div onunknownevent="alert(&quot;hack&quot;)" />,
        1,
      );
      expect(e.getAttribute('onunknownevent')).toBe(null);
    });
    itRenders('custom attribute named `on`', async render => {
      const e = await render(<div on="tap:do-something" />);
      expect(e.getAttribute('on')).toEqual('tap:do-something');
    });
  });
  describe('elements and children', function() {
    function expectNode(node, type, value) {
      expect(node).not.toBe(null);
      expect(node.nodeType).toBe(type);
      expect(node.nodeValue).toMatch(value);
    }
    function expectTextNode(node, text) {