[DS-1833] Collection content source harvesting test does not work with ORE

The harvesting feature on collections does not resolve the ORE namespace properly when testing. 
For the test the ORE namespace is hardcoded to "http://www.openarchives.org/ore/terms/" while the actual import has a method that will determine the ORE namespace based on configuration (with a fallback on Atom). 

The default namespace for ORE in DSpace OAI is "http://www.w3.org/2005/Atom" so even though the test says that the OAI repo doesn't support ORE the import will work. 


You can test this issue by attempting to harvest demo.dspace.org (this offers ORE by default) (http://demo.dspace.org/oai/request?verb=ListMetadataFormats)

This pull request ensures consistency when it comes down to the ORE metadataFormat.