Consider making this formatting string a method since it wasn't obvious what this was doing at first.

```ruby
def write!
  ...
          content << entry.columns.map.with_index do |column_value, i|
            right_pad(column_value, @column_lengths[i])
  ...
end

def  right_pad(column_value, desired_length)
  "%-#{desired_length}s" % column_value
end
```