DATAMONGO-925 - Improveed custom field-naming-strategy handling in MappingMongoConverterParser.

Relaxed precondition for fieldNameStrategy setting to allow custom-field-naming strategies to be set.
MappingMongoConverterParser incorrectly rejected a configured field-naming-strategy-ref if abbreviate-field-names was not set to an empty string in XML configuration.

Original pull request: #177.