use native EventSource if available

This ensures native EventSource will be used if it's available.

As EventSource + CORS is now [more widely supported](https://github.com/Fyrd/caniuse/blob/master/features-json/eventsource.json#L22), this seems like a better default.

Currently the tests just exit if native support is detected. Probably a better solution would be to (somehow) force the tests to use the polyfill version.