**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.
parentNamespace: string,
  ): Element {
    
    
    var ownerDocument: Document = rootContainerElement.nodeType ===
      DOCUMENT_NODE
      ? (rootContainerElement: any)
      : rootContainerElement.ownerDocument;
    var domElement: Element;
    var namespaceURI = parentNamespace;
    if (namespaceURI === HTML_NAMESPACE) {
      namespaceURI = getIntrinsicNamespace(type);
    }
    if (namespaceURI === HTML_NAMESPACE) {
      if (__DEV__) {
        var isCustomComponentTag = isCustomComponent(type, props);
        
        
        warning(
          isCustomComponentTag || type === type.toLowerCase(),
          '<%s /> is using uppercase HTML. Always use lowercase HTML tags ' +
            'in React.',
          type,
        );
      }
      if (type === 'script') {
        
        
        var div = ownerDocument.createElement('div');
        div.innerHTML = '<script><' + '/script>';