Add missing assoc test to Ref
Add missing assoc test to Ref

Mdm::ref was misisng an association spec

rapid7/metasploit-model,rapid7/metasploit-cache,rapid7/metasploit-cache,bcook-r7/metasploit_data_models,farias-r7/metasploit_data_models,rapid7/metasploit-model,rapid7/metasploit_data_models,rapid7/metasploit-cache,rapid7/metasploit-cache,farias-r7/metasploit_data_models,rapid7/metasploit_data_models,bcook-r7/metasploit_data_models,farias-r7/metasploit_data_models,bcook-r7/metasploit_data_models,rapid7/metasploit_data_models,rapid7/metasploit-model