MP Rest Client CompletionStages from async methods are never completed

Manish Kumar first reported this issue in a [Twitter post|https://twitter.com/manish2aug/status/1155558627593379842] and then opened issue 208 on the MP Rest Client spec page.

He provided an example that shows that the CompletionStage never completes, leaving his test case hung.  See the example here:

https://gist.github.com/manish2aug/b4133b9fbedc2175e15b303a0682b552

It should be possible to fix this with a change to the MPRestClientCallback.