OAIHarvest: fix identifier parsing

I screwed up something in the OAI harvest deamon in regards to one-shot harvesting of identifiers. Basically anything with a `/` got an arXiv prefix. :-1:  

Here I am trying to fix my mistake (by simply dropping the old arXiv ID support) and hoping to recover my honour by adding tests. :dancer: 

Thanks @switowski for reporting it.