**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.
const errorToThrow = firstUncaughtError;
     We're done performing work. Time to clean up.
    isPerformingWork = false;
    deadlineHasExpired = false;
    didFatal = false;
    firstUncaughtError = null;
    capturedErrors = null;
    failedBoundaries = null;
    nextRenderedTree = null;
    nestedUpdateCount = 0;
    if (__DEV__) {
      stopWorkLoopTimer();
    }
     It's safe to throw any unhandled errors.
    if (errorToThrow !== null) {
      throw errorToThrow;
    }
  }
  
  function captureError(failedWork: Fiber, error: mixed): Fiber | null {
    
    ReactCurrentOwner.current = null;
    if (__DEV__) {
      ReactDebugCurrentFiber.resetCurrentFiber();
    }
     Search for the nearest error boundary.
    let boundary: Fiber | null = null;
     Passed to logCapturedError()
    let errorBoundaryFound: boolean = false;
    let willRetry: boolean = false;