BUG: Fix missed cache invalidations on assignment.
BUG: Fix missed cache invalidations on assignment.

_position_values needs to be invalidated on every assignment.

jimgoo/zipline-fork,jordancheah/zipline,stkubr/zipline,YuepengGuo/zipline,bartosh/zipline,aajtodd/zipline,wilsonkichoi/zipline,nborggren/zipline,ronalcc/zipline,ChinaQuants/zipline,dmitriz/zipline,keir-rex/zipline,humdings/zipline,alphaBenj/zipline,enigmampc/catalyst,jimgoo/zipline-fork,alphaBenj/zipline,CDSFinance/zipline,florentchandelier/zipline,cmorgan/zipline,michaeljohnbennett/zipline,StratsOn/zipline,semio/zipline,dkushner/zipline,wilsonkichoi/zipline,zhoulingjun/zipline,keir-rex/zipline,sketchytechky/zipline,semio/zipline,otmaneJai/Zipline,grundgruen/zipline,ChinaQuants/zipline,AlirezaShahabi/zipline,euri10/zipline,dmitriz/zipline,umuzungu/zipline,humdings/zipline,kmather73/zipline,Scapogo/zipline,wubr2000/zipline,bartosh/zipline,otmaneJai/Zipline,florentchandelier/zipline,kmather73/zipline,dkushner/zipline,morrisonwudi/zipline,chrjxj/zipline,chrjxj/zipline,MonoCloud/zipline,quantopian/zipline,AlirezaShahabi/zipline,iamkingmaker/zipline,magne-max/zipline-ja,jordancheah/zipline,enigmampc/catalyst,DVegaCapital/zipline,iamkingmaker/zipline,sketchytechky/zipline,CDSFinance/zipline,michaeljohnbennett/zipline,StratsOn/zipline,Scapogo/zipline,magne-max/zipline-ja,morrisonwudi/zipline,gwulfs/zipline,zhoulingjun/zipline,joequant/zipline,joequant/zipline,YuepengGuo/zipline,gwulfs/zipline,euri10/zipline,nborggren/zipline,MonoCloud/zipline,cmorgan/zipline,stkubr/zipline,wubr2000/zipline,DVegaCapital/zipline,umuzungu/zipline,grundgruen/zipline,quantopian/zipline,ronalcc/zipline,aajtodd/zipline