**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.
fiber: Fiber,
    forceAsync: boolean,
  ): PriorityLevel {
    let priorityLevel = priorityContext;
    if (priorityLevel === NoWork) {
      if (
        !useSyncScheduling ||
        fiber.internalContextTag & AsyncUpdates ||
        forceAsync
      ) {
        priorityLevel = LowPriority;
      } else {
        priorityLevel = SynchronousPriority;
      }
    }
     
     
    if (
      priorityLevel === SynchronousPriority &&
      (isPerformingWork || isBatchingUpdates)
    ) {
      return TaskPriority;
    }
    return priorityLevel;
  }
  function scheduleErrorRecovery(fiber: Fiber) {