arxiv: OAI2 API usage and status code addition

* Changes the arXiv import source to OAI2 url.  (addresses #1866)

* Adds status code to API request.

* Updates existing unit tests and adds new ones: unsupported
  versioning and malformed.

* It's able to get more info than before.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>