**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.
if (isUnmounting) {
            return null;
          }
           If we're in the commit phase, we should check to see if
           this boundary already captured an error during this commit.
           This case exists because multiple errors can be thrown during
           a single commit without interruption.
          if (
            commitPhaseBoundaries !== null &&
            (commitPhaseBoundaries.has(node) ||
              (node.alternate !== null &&
                commitPhaseBoundaries.has(node.alternate)))
          ) {
            
            return null;
          }
           The error should propagate to the next boundary -— we keep looking.
          boundary = null;
          willRetry = false;
        }
        node = node.return;
      }
    }
    if (boundary !== null) {
       
       
      if (failedBoundaries === null) {