**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.
} else {
      value = fn(props, context);
    }
    
    workInProgress.effectTag |= PerformedWork;
    if (
      typeof value === 'object' &&
      value !== null &&
      typeof value.render === 'function'
    ) {
      
      workInProgress.tag = ClassComponent;
      
      
      
      const hasContext = pushContextProvider(workInProgress);
      adoptClassInstance(workInProgress, value);
      mountClassInstance(workInProgress, priorityLevel);
      return finishClassComponent(current, workInProgress, true, hasContext);
    } else {
      
      workInProgress.tag = FunctionalComponent;
      if (__DEV__) {
        const Component = workInProgress.type;
        if (Component) {
          warning(
            !Component.childContextTypes,
            '%s(...): childContextTypes cannot be defined on a functional component.',
            Component.displayName || Component.name || 'Component',
          );