**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.
let isBatchingUpdates: boolean = false;
  
  
  let isUnbatchingUpdates: boolean = false;
  
  let nextUnitOfWork: Fiber | null = null;
  let nextPriorityLevel: PriorityLevel = NoWork;
  
  let nextEffect: Fiber | null = null;
  let pendingCommit: Fiber | null = null;
  
  let nextScheduledRoot: FiberRoot | null = null;
  let lastScheduledRoot: FiberRoot | null = null;
  
  let isCallbackScheduled: boolean = false;
  
  
  let capturedErrors: Map<Fiber, CapturedError> | null = null;
  
  
  
  
  let failedBoundaries: Set<Fiber> | null = null;
   Error boundaries that captured an error during the current commit.
  let commitPhaseBoundaries: Set<Fiber> | null = null;
  let firstUncaughtError: mixed | null = null;
  let didFatal: boolean = false;
  let isCommitting: boolean = false;
  let isUnmounting: boolean = false;
   Use these to prevent an infinite loop of nested updates