More enhancements to Mongo names and tests.

1. On names, the mongo identifier of the target entity will be used if direct relationship is named after the entity.
2. On tests, the functionality above is included in MongoNamesTest (and in the integrated tests .srm model).

Also, for clarity of the relationships, entity names will be separated with "," when composing the relationship name (before, they were spearated by " " (space)).