fix: allow for getUpstream flow for empty wsUpstream option

[jcbain]

Resolves #333

This fix makes using wsUpstream: '' with a set replyOptions.getUpstream consistent with the established flow of using upstream: '' with a getUpstream set. To do so, just some added logic to the plugin initialization phase as well as updating the WebsocketProxy constructor logic. Note, I maintained the need for the websocket flag needing to be set to true to remain consistent with the proxying of websockets currently.

This should also allow for dynamically setting a websocket upstream through the use of the websocket options provided, which should be a bit more clear for those wanting that behavior.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm