**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`
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');
      });
      itRenders('no null styles', async render => {
        const e = await render(<div style={{color: null, width: '30px'}} />);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('30px');
      });
      itRenders('no empty styles', async render => {
        const e = await render(<div style={{color: null, width: null}} />);
        expect(e.style.color).toBe('');
        expect(e.style.width).toBe('');
        expect(e.hasAttribute('style')).toBe(false);
      });
      itRenders('unitless-number rules with prefixes', async render => {
        const {style} = await render(
          <div
            style={{
              lineClamp: 10,
              WebkitLineClamp: 10,
              MozFlexGrow: 10,