**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.
testDocument = document.implementation.createHTMLDocument();
    }
    var testElement = parent.namespaceURI === HTML_NAMESPACE
      ? testDocument.createElement(parent.tagName)
      : testDocument.createElementNS(
          (parent.namespaceURI: any),
          parent.tagName,
        );
    testElement.innerHTML = html;
    return testElement.innerHTML;
  };
}
function ensureListeningTo(rootContainerElement, registrationName) {
  var isDocumentOrFragment =
    rootContainerElement.nodeType === DOCUMENT_NODE ||
    rootContainerElement.nodeType === DOCUMENT_FRAGMENT_NODE;
  var doc = isDocumentOrFragment
    ? rootContainerElement
    : rootContainerElement.ownerDocument;
  listenTo(registrationName, doc);
}
var mediaEvents = {
  topAbort: 'abort',
  topCanPlay: 'canplay',
  topCanPlayThrough: 'canplaythrough',
  topDurationChange: 'durationchange',
  topEmptied: 'emptied',
  topEncrypted: 'encrypted',
  topEnded: 'ended',
  topError: 'error',