**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.
ensureListeningTo(rootContainerElement, 'onChange');
        break;
    }
    assertValidProps(tag, rawProps, getCurrentFiberOwnerName);
    if (__DEV__) {
      var extraAttributeNames: Set<string> = new Set();
      var attributes = domElement.attributes;
      for (var i = 0; i < attributes.length; i++) {
        var name = attributes[i].name.toLowerCase();
        switch (name) {
          
          case 'data-reactroot':
            break;
          
          
          case 'value':
            break;
          case 'checked':
            break;
          case 'selected':
            break;
          default:
            extraAttributeNames.add(attributes[i].name);
        }
      }
    }
    var updatePayload = null;
    for (var propKey in rawProps) {
      if (!rawProps.hasOwnProperty(propKey)) {