def to_datastream(*a)
      constraints = a.first && a.first[:constraints] ? [a.first[:constraints]] : []
      to_image(*a).to_datastream(*constraints)
    end
def to_datastream(*a)
      constraints = a.first && a.first[:constraints] ? [a.first[:constraints]] : []
      to_image(*a).to_datastream(*constraints)
    end
Create a ChunkyPNG::Datastream containing the barcode image

 :constraints - Value is passed on to ChunkyPNG::Image#to_datastream
                E.g. to_datastream(:constraints => {:color_mode => ChunkyPNG::COLOR_GRAYSCALE})