**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.
startWorkTimer,
    stopWorkTimer,
    stopFailedWorkTimer,
    startWorkLoopTimer,
    stopWorkLoopTimer,
    startCommitTimer,
    stopCommitTimer,
    startCommitHostEffectsTimer,
    stopCommitHostEffectsTimer,
    startCommitLifeCyclesTimer,
    stopCommitLifeCyclesTimer,
  } = require('ReactDebugFiberPerf');
  var warnAboutUpdateOnUnmounted = function(
    instance: React$ComponentType<any>,
  ) {
    const ctor = instance.constructor;
    warning(
      false,
      'Can only update a mounted or mounting component. This usually means ' +
        'you called setState, replaceState, or forceUpdate on an unmounted ' +
        'component. This is a no-op.\n\nPlease check the code for the ' +
        '%s component.',
      (ctor && (ctor.displayName || ctor.name)) || 'ReactClass',
    );
  };
  var warnAboutInvalidUpdates = function(instance: React$ComponentType<any>) {
    switch (ReactDebugCurrentFiber.phase) {
      case 'getChildContext':
        warning(
          false,