Add test for polymorphic associations.
Add test for polymorphic associations.

kiwih/gorm,arthurpro/gorm,ivpusic/gorm,awakenetworks/gorm,xaionaro/gorm,arthurpro/gorm,unravelin/gorm,sunfmin/gorm,seatgeek/gorm,kimiby/gorm,ilgooz/gorm,bpizzi/gorm,lscieux/gorm,liudanking/gorm,pariz/gorm,jinzhu/gorm,awakenetworks/gorm,liudanking/gorm,ModelRocket/gorm,pariz/gorm,jinzhu/gorm,bpizzi/gorm,aspcartman/gorm,Acidburn0zzz/gorm,xinhuang327/gorm,OscaRoa/gorm,andviro/gorm,trigrass2/gorm,armiller/gorm,postmates/gorm,ngorm/ngorm,gernest/ngorm,ch3lo/gorm,ivpusic/gorm,xinhuang327/gorm,wallclockbuilder/gorm,sunfmin/gorm,sidorenko-ruslan/gorm,acdgfh/gorm,d4l3k/gorm,jaytaylor/gorm,nkovacs/gorm,KevinSjoberg/gorm,ModelRocket/gorm,sdhjl2000/gorm,seatgeek/gorm,sdhjl2000/gorm,senseobservationsystems/gorm,ulule/gorm,NOX73/gorm,embroker/gorm,wallclockbuilder/gorm,sidorenko-ruslan/gorm,aperera/gorm,sulnedinfind/gorm,Twister915/gorm,javierprovecho/gorm,jmptrader/gorm,acdgfh/gorm,andviro/gorm,ulule/gorm,unravelin/gorm,ch3lo/gorm,yibin001/gorm,kimiby/gorm,Acidburn0zzz/gorm,Twister915/gorm,otiai10/gorm,kiwih/gorm,nkovacs/gorm,yibin001/gorm,postmates/gorm,aperera/gorm,embroker/gorm,OscaRoa/gorm,lscieux/gorm,sulnedinfind/gorm,trigrass2/gorm,xaionaro/gorm,javierprovecho/gorm,ilgooz/gorm,otiai10/gorm,senseobservationsystems/gorm,d4l3k/gorm,jmptrader/gorm,KevinSjoberg/gorm,aspcartman/gorm,NOX73/gorm,jaytaylor/gorm,armiller/gorm