**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.
We've run out of time. Exit.
          break loop;
        case NoWork:
           No work left. We can exit.
          break loop;
        default:
          invariant(
            false,
            'Switch statement should be exhuastive. ' +
              'This error is likely caused by a bug in React. Please file an issue.',
          );
      }
    } while (true);
  }
  function performWorkCatchBlock(
    failedWork: Fiber,
    boundary: Fiber,
    minPriorityLevel: PriorityLevel,
    deadline: Deadline | null,
  ) {
    
    
    
    unwindContexts(failedWork, boundary);
    
    
    
    
    
    nextUnitOfWork = performFailedUnitOfWork(boundary);