**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.
'setState(...): Cannot call setState() inside getChildContext()',
        );
        break;
      case 'render':
        warning(
          false,
          'Cannot update during an existing state transition (such as within ' +
            "`render` or another component's constructor). Render methods should " +
            'be a pure function of props and state; constructor side-effects are ' +
            'an anti-pattern, but can be moved to `componentWillMount`.',
        );
        break;
    }
  };
}
var timeHeuristicForUnitOfWork = 1;
module.exports = function<T, P, I, TI, PI, C, CX, PL>(
  config: HostConfig<T, P, I, TI, PI, C, CX, PL>,
) {
  const hostContext = ReactFiberHostContext(config);
  const hydrationContext: HydrationContext<C> = ReactFiberHydrationContext(
    config,
  );
  const {popHostContainer, popHostContext, resetHostContainer} = hostContext;
  const {beginWork, beginFailedWork} = ReactFiberBeginWork(
    config,
    hostContext,
    hydrationContext,
    scheduleUpdate,
    getPriorityContext,