def []=(key, value)
      if value.nil?
        @values.delete(key)
      else
        bmp_def              = _get_definition key
        bmp_def.value        = value
        @values[bmp_def.bmp] = bmp_def
      end
    end
def []=(key, value)
      if value.nil?
        @values.delete(key)
      else
        bmp_def              = _get_definition key
        bmp_def.value        = value
        @values[bmp_def.bmp] = bmp_def
      end
    end
Set a field in this message, `key` is either the
 bmp number or it's name.
 ===Example

    mes = BlaBlaMessage.new
    mes[2]=47474747                          # bmp 2 is generally the PAN
    mes["Primary Account Number"]=47474747   # if thats what you called the field in Message.bmp.