**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`
let namespace;
    const nodeType = rootContainerInstance.nodeType;
    switch (nodeType) {
      case DOCUMENT_NODE:
      case DOCUMENT_FRAGMENT_NODE: {
        type = nodeType === DOCUMENT_NODE ? '#document' : '#fragment';
        let root = (rootContainerInstance: any).documentElement;
        namespace = root ? root.namespaceURI : getChildNamespace(null, '');
        break;
      }
      default: {
        const container: any = nodeType === COMMENT_NODE
          ? rootContainerInstance.parentNode
          : rootContainerInstance;
        const ownNamespace = container.namespaceURI || null;
        type = container.tagName;
        namespace = getChildNamespace(ownNamespace, type);
        break;
      }
    }
    if (__DEV__) {
      const validatedTag = type.toLowerCase();
      const ancestorInfo = updatedAncestorInfo(null, validatedTag, null);
      return {namespace, ancestorInfo};
    }
    return namespace;
  },
  getChildHostContext(
    parentHostContext: HostContext,
    type: string,