Revert "Turn unnamed lists into BSON arrays instead of objects."

Reverts mongosoup/rmongodb#49

> ll <- jsonlite:::fromJSON('{"$group":{"_id":"$state", "totalPop":{"$sum":"$pop"}}}')
> 
> mongo.bson.from.list(ll)
Error in mongo.bson.from.list(ll) : 
  STRING_ELT() can only be applied to a 'character vector', not a 'char'

not working!