Add unit test to show that the swagger generated POST request is actually valid

[codefromthecrypt]

In #51, we learned there are some subtleties about how openapi/swagger generates requests. These may be generator specific, but it could help to have something in a test that shows the POST request actually implies a ListOfSpans payload as opposed to a "spans" -> ListOfSpans. This can be more helpful as we consider moving forward with swagger versions.

cc @drolando in case you know the swag(ger)