Don't use custom separator to serialize lists of parameters in OAI-PMH publication service

The OAI-PMH publication service implementation uses the custom separator OaiPmhPublicationServiceImpl.SEPARATOR (';;') to separate list items when serializing parameter lists.

While this work in practise based on the assumption that no list items ever contain that separator, it is not proofed that this assumption does hold.

Instead of having code making unproofed assumption (or attempts to proof the assumptions) it would be better to properly serialize the parameter lists (e.g. using JSON).