**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.
var firstChild = ((div.firstChild: any): HTMLScriptElement);
        domElement = div.removeChild(firstChild);
      } else if (typeof props.is === 'string') {
        
        domElement = ownerDocument.createElement(type, {is: props.is});
      } else {
        
        
        
        domElement = ownerDocument.createElement(type);
      }
    } else {
      domElement = ownerDocument.createElementNS(namespaceURI, type);
    }
    if (__DEV__) {
      if (namespaceURI === HTML_NAMESPACE) {
        if (
          !isCustomComponentTag &&
          Object.prototype.toString.call(domElement) ===
            '[object HTMLUnknownElement]' &&
          !Object.prototype.hasOwnProperty.call(warnedUnknownTags, type)
        ) {
          warnedUnknownTags[type] = true;
          warning(
            false,
            'The tag <%s> is unrecognized in this browser. ' +
              'If you meant to render a React component, start its name with ' +
              'an uppercase letter.',
            type,