**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.
OffscreenPriority,
} = require('ReactPriorityLevel');
var {AsyncUpdates} = require('ReactTypeOfInternalContext');
var {
  PerformedWork,
  Placement,
  Update,
  PlacementAndUpdate,
  Deletion,
  ContentReset,
  Callback,
  Err,
  Ref,
} = require('ReactTypeOfSideEffect');
var {
  HostRoot,
  HostComponent,
  HostPortal,
  ClassComponent,
} = require('ReactTypeOfWork');
var {getUpdatePriority} = require('ReactFiberUpdateQueue');
var {resetContext} = require('ReactFiberContext');
var invariant = require('fbjs/lib/invariant');
if (__DEV__) {
  var warning = require('fbjs/lib/warning');
  var ReactFiberInstrumentation = require('ReactFiberInstrumentation');
  var ReactDebugCurrentFiber = require('ReactDebugCurrentFiber');
  var {
    recordEffect,
    recordScheduleUpdate,