OAIHarvest: fix identifier parsing

* Fixes the identifier parsing that was too happy about
  adding arXiv prefix to identifiers specified via CLI or
  web interface when harvesting.

* Adds tests for the identifier parsing.

Reported-by: Sebastian Witowski <sebastian.witowski@cern.ch>
Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>