Add origin property to MessageEvent

[clue]

The MessageEvent should have an origin property that references the URL of the EventSource object (after following any redirects): https://html.spec.whatwg.org/multipage/comms.html#dom-messageevent-origin

Depends on clue/reactphp-buzz#129 in order to be able to access the last effective URL.

[clue]

For the reference: This now depends on reactphp/http#388 after the HTTP client has been updated via #13.

[clue]

I still think that this feature makes perfect sense 👍 However, there are currently no immediate plans to build this from my end (no demand at the moment and more important outstanding issues currently), so I'm closing this for now.

We welcome contributions, reach out if you want to support this project 👍