**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 (console.error.calls && console.error.calls.reset) {
    console.error.calls.reset();
  } else {
    spyOn(console, 'error');
  }
  const result = await fn();
  if (
    console.error.calls.count() !== count &&
    console.error.calls.count() !== 0
  ) {
    console.log(
      `We expected ${count} warning(s), but saw ${console.error.calls.count()} warning(s).`,
    );
    if (console.error.calls.count() > 0) {
      console.log(`We saw these warnings:`);
      for (var i = 0; i < console.error.calls.count(); i++) {
        console.log(console.error.calls.argsFor(i)[0]);
      }
    }
  }
  expectDev(console.error.calls.count()).toBe(count);
  return result;
}
function renderIntoDom(reactElement, domElement, forceHydrate, errorCount = 0) {