MAINT: Slice into returns containers instead of using .valid()
MAINT: Slice into returns containers instead of using .valid()

The slicing syntax is more explicit about declaring:
'get all returns up until the current dt'.

Also, protects against NaNs that occur before the current dt
being silently ignored.
i.e. the *_returns_cont series *should* have values from start
to current dt, but the .valid() call was occluding a bug where
it wasn't.

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