**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.
failedWork,
            boundary,
            minPriorityLevel,
            deadline,
          );
          error = null;
        } catch (e) {
          didError = true;
          error = e;
          continue;
        }
      }
      
      break;
    }
     Reset the priority context to its previous value.
    priorityContext = previousPriorityContext;
     If we're inside a callback, set this to false, since we just flushed it.
    if (deadline !== null) {
      isCallbackScheduled = false;
    }
     If there's remaining async work, make sure we schedule another callback.
    if (nextPriorityLevel > TaskPriority && !isCallbackScheduled) {
      scheduleDeferredCallback(performDeferredWork);
      isCallbackScheduled = true;
    }