Subscribing to events using CoAP returns `undefined`

[JKRhb]

As discovered by @relu91, there is currently a bug in the CoAP binding implementation that leads to servers returning undefined when the initial observe GET request is received. The issue is being caused by this line, leading to the request being responded to before the event handler passes a value:

node-wot/packages/binding-coap/src/coap-server.ts

Line 887 in 450ebd9

res.end();