**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('no className prop with null value', async render => {
        const e = await render(<div className={null} />);
        expect(e.hasAttribute('className')).toBe(false);
      });
      itRenders('badly cased className with a warning', async render => {
        const e = await render(<div classname="test" />, 1);
        expect(e.hasAttribute('class')).toBe(false);
        expect(e.hasAttribute('classname')).toBe(true);
      });
      itRenders(
        'className prop when given the alias with a warning',
        async render => {
          const e = await render(<div class="test" />, 1);
          expect(e.className).toBe('test');
        },
      );
      itRenders(
        'className prop when given a badly cased alias',
        async render => {
          const e = await render(<div cLASs="test" />, 1);
          expect(e.className).toBe('test');
        },
      );
      itRenders('class for custom elements', async render => {
        const e = await render(<div is="custom-element" class="test" />, 0);
        expect(e.getAttribute('class')).toBe('test');
      });
      itRenders('className for custom elements', async render => {
        const e = await render(<div is="custom-element" className="test" />, 0);
        expect(e.getAttribute('className')).toBe('test');