arxiv: improvements to Flask extension

* Changes the arXiv import source to oai2 url.

* Updates unit tests.

* It's able to get more info than before.

Signed-off-by: Pedro Gaudêncio <pedro.gaudencio@cern.ch>
Ok. Maybe we should be more friendly in our wrapper and return 404 code for `notfound`. @pedrogaudencio What do you think?