MAINT: Reverses polarity on keep transactions default.
MAINT: Reverses polarity on keep transactions default.

So that transactions are kept by default.

This prepares for the addition of the serialize flag added by
@fawce.

Setting the default to True, so that the flags will be aligned.

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