txn: fix incorrect assumption on PurgeMissing test
txn: fix incorrect assumption on PurgeMissing test

raymondjacobson/mgo,jjz/mgo,DroiTaipei/mgo,Yottaa/mgo.v2,mjs/mgo,tmc/mgo,deafgoat/mgo,deafgoat/mgo,mcuadros/mgo,velocity-toolkit/mgo,jackspirou/mgo,tchajed/mgo,tchajed/mgo,jackspirou/mgo,Danzeer/mgo,hexadecy/mgo,Yottaa/mgo.v2,tchajed/mgo,Yottaa/mgo.v2,mjs/mgo,jjz/mgo,go-mgo/travis-test,dmage/mgo,mjs/mgo,velocity-toolkit/mgo,Danzeer/mgo,jjz/mgo,tchajed/mgo,gabrielrussell/mgo,DroiTaipei/mgo,gabrielrussell/mgo,hexadecy/mgo,deafgoat/mgo,lucasjo/mgo,raymondjacobson/mgo,raymondjacobson/mgo,raymondjacobson/mgo,velocity-toolkit/mgo,jackspirou/mgo,go-mgo/travis-test,mjs/mgo,deafgoat/mgo,tmc/mgo,SocialExplorerFork/mgo,henrylee2cn/mgo,Danzeer/mgo,dmage/mgo,DroiTaipei/mgo,mcuadros/mgo,hexadecy/mgo,henrylee2cn/mgo,SocialExplorerFork/mgo,tmc/mgo,go-mgo/travis-test,velocity-toolkit/mgo,DroiTaipei/mgo,jackspirou/mgo,go-mgo/travis-test,SocialExplorerFork/mgo,mcuadros/mgo,hexadecy/mgo,gabrielrussell/mgo,henrylee2cn/mgo,Danzeer/mgo,lucasjo/mgo,jjz/mgo,dmage/mgo,lucasjo/mgo,SocialExplorerFork/mgo,Yottaa/mgo.v2,dmage/mgo,gabrielrussell/mgo,tmc/mgo,mcuadros/mgo,lucasjo/mgo,henrylee2cn/mgo,tchajed/mgo