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