Makes FetchEventSource compatible with webworker

[stefdev49]

Hello,

these changes (somehow similar to https://github.com/Azure/fetch-event-source/pull/8) are the minimal ones I had to do to get FetchEventSource working in a web worker context. I've also added some unit tests to validate these changes.

With it one can use FetchEventSource in a WebWorker by settings these parameters :

fetchEventSource('https://myurl', {
    openWhenHidden: true,
    fetch: self.fetch,
    .....

[ghost]

All CLA requirements met.