**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.
rootContainerElement: Element | Document,
  nextProps: Object,
  isCustomComponentTag: boolean,
): void {
  for (var propKey in nextProps) {
    if (!nextProps.hasOwnProperty(propKey)) {
      continue;
    }
    var nextProp = nextProps[propKey];
    if (propKey === STYLE) {
      if (__DEV__) {
        if (nextProp) {
          
          
          Object.freeze(nextProp);
        }
      }
      
      CSSPropertyOperations.setValueForStyles(domElement, nextProp);
    } else if (propKey === DANGEROUSLY_SET_INNER_HTML) {
      var nextHtml = nextProp ? nextProp[HTML] : undefined;
      if (nextHtml != null) {
        setInnerHTML(domElement, nextHtml);
      }
    } else if (propKey === CHILDREN) {
      if (typeof nextProp === 'string') {
        setTextContent(domElement, nextProp);
      } else if (typeof nextProp === 'number') {
        setTextContent(domElement, '' + nextProp);
      }