**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.
return;
    }
    didWarnInvalidHydration = true;
    var names = [];
    attributeNames.forEach(function(name) {
      names.push(name);
    });
    warning(false, 'Extra attributes from the server: %s', names);
  };
  var warnForInvalidEventListener = function(registrationName, listener) {
    warning(
      false,
      'Expected `%s` listener to be a function, instead got a value of `%s` type.%s',
      registrationName,
      typeof listener,
      getCurrentFiberStackAddendum(),
    );
  };
  var testDocument;
  
  
  var normalizeHTML = function(parent: Element, html: string) {
    if (!testDocument) {