**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.
false,
        'Maximum update depth exceeded. This can happen when a ' +
          'component repeatedly calls setState inside componentWillUpdate or ' +
          'componentDidUpdate. React limits the number of nested updates to ' +
          'prevent infinite loops.',
      );
    }
    if (!isPerformingWork && priorityLevel <= nextPriorityLevel) {
       
       
       
      nextUnitOfWork = null;
    }
    if (__DEV__) {
      if (!isErrorRecovery && fiber.tag === ClassComponent) {
        const instance = fiber.stateNode;
        warnAboutInvalidUpdates(instance);
      }
    }
    let node = fiber;
    let shouldContinue = true;
    while (node !== null && shouldContinue) {
       
       
       
      shouldContinue = false;
      if (
        node.pendingWorkPriority === NoWork ||
        node.pendingWorkPriority > priorityLevel
      ) {