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.
as.data.frame is a generic function already, so we don't need to setGeneric. BTW, it should be standardGeneric("as.data.frame") to be correct. Pls test it before you commit your changes, thanks.