**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.
render() {
              return undefined;
            }
          }
          await render(<UndefinedComponent />, 1);
        },
        ReactDOMFeatureFlags.useFiber
          ? 'UndefinedComponent(...): Nothing was returned from render. ' +
              'This usually means a return statement is missing. Or, to ' +
              'render nothing, return null.'
          : 'A valid React element (or null) must be returned. ' +
              'You may have returned undefined, an array or some other invalid object.',
      );
      itThrowsWhenRendering(
        'a function returning an object',
        async render => {
          const ObjectComponent = () => ({x: 123});
          await render(<ObjectComponent />, 1);
        },
        ReactDOMFeatureFlags.useFiber
          ? 'Objects are not valid as a React child (found: object with keys ' +
              '{x}). If you meant to render a collection of children, use ' +
              'an array instead.'
          : 'A valid React element (or null) must be returned. ' +
              'You may have returned undefined, an array or some other invalid object.',
      );
      itThrowsWhenRendering(
        'a class returning an object',
        async render => {
          class ObjectComponent extends React.Component {