**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.
let errorBoundaryName: string | null = null;
     Host containers are a special case. If the failed work itself is a host
     container, then it acts as its own boundary. In all other cases, we
     ignore the work itself and only search through the parents.
    if (failedWork.tag === HostRoot) {
      boundary = failedWork;
      if (isFailedBoundary(failedWork)) {
        
        
        
        didFatal = true;
      }
    } else {
      let node = failedWork.return;
      while (node !== null && boundary === null) {
        if (node.tag === ClassComponent) {
          const instance = node.stateNode;
          if (typeof instance.componentDidCatch === 'function') {
            errorBoundaryFound = true;
            errorBoundaryName = getComponentName(node);
            
            boundary = node;
            willRetry = true;
          }
        } else if (node.tag === HostRoot) {
          
          boundary = node;
        }
        if (isFailedBoundary(node)) {