**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.
};
export type HandleErrorInfo = {
  componentStack: string,
};
var {popContextProvider} = require('ReactFiberContext');
const {reset} = require('ReactFiberStack');
var {
  getStackAddendumByWorkInProgressFiber,
} = require('ReactFiberComponentTreeHook');
var {logCapturedError} = require('ReactFiberErrorLogger');
var {
  invokeGuardedCallback,
  hasCaughtError,
  clearCaughtError,
} = require('ReactErrorUtils');
var ReactFiberBeginWork = require('ReactFiberBeginWork');
var ReactFiberCompleteWork = require('ReactFiberCompleteWork');
var ReactFiberCommitWork = require('ReactFiberCommitWork');
var ReactFiberHostContext = require('ReactFiberHostContext');
var ReactFiberHydrationContext = require('ReactFiberHydrationContext');
var {ReactCurrentOwner} = require('ReactGlobalSharedState');
var getComponentName = require('getComponentName');
var {createWorkInProgress, largerPriority} = require('ReactFiber');
var {onCommitRoot} = require('ReactFiberDevToolsHook');
var {
  NoWork,
  SynchronousPriority,
  TaskPriority,
  HighPriority,
  LowPriority,