Fix head bug

when the ddf contains only one column, there exits errors:

Error in paste("as", type, sep = ".") : 
  cannot coerce type 'closure' to vector of type 'character'

ADD dataFrame method to choose selected columns as R dataFrame.
Each function should do only one thing. The user can do subsetting first and then fetch to local -> as.data.frame(ddf[,"mpg"]).