allow extra mongoDB fields unkown to persistent
allow extra mongoDB fields unkown to persistent

requested at:
http://stackoverflow.com/questions/8209139/mapping-to-legacy-mongodb-store

andrewthad/persistent,yesodweb/persistent,meteficha/persistent,superduper/persistent,rdnetto/persistent,greydot/persistent,creichert/persistent,gbwey/persistent,naushadh/persistent,greydot/persistent,gbwey/persistentold,psibi/persistent,meteficha/persistent,andrewthad/persistent,mitchellwrosen/persistent,superduper/persistent,mkscrg/persistent,jasonzoladz/persistent,jasonzoladz/persistent,gbwey/persistentold,mitchellwrosen/persistent,jcristovao/persistent,meteficha/persistent,pseudonom/persistent,plow-technologies/persistent,greydot/persistent,erikd/persistent,gbwey/persistent,junjihashimoto/persistent,jasonzoladz/persistent,superduper/persistent,jcristovao/persistent,mitchellwrosen/persistent,bitemyapp/persistent,andrewthad/persistent,junjihashimoto/persistent,mkscrg/persistent,jcristovao/persistent,nakaji-dayo/persistent,nakaji-dayo/persistent,pseudonom/persistent,paul-rouse/persistent,rdnetto/persistent,mkscrg/persistent,gbwey/persistent,nakaji-dayo/persistent