Add tests for JdkHttpOaiClient

[poikilotherm]

Currently, Dataverse does not make use of the service provider part of this library. Leaving this for a later exercise.

[poikilotherm]

Dataverse now does use this, as we saw with #118. Maybe we should think about this one again...?

[poikilotherm]

This might be related to an implementation for #141, as we could use WireMock to try different OAI-PMH responses from out in the wild here.