Fix separator detection not working for one column csv files

[mikekreeki]

Passing col_sep: nil option to CSV constructor causes CSV to separate each character as a separate column.

With CSV content as follows:

name
productboard

Parsed row before the fix (incorrect):

['p', 'r', 'o', 'd', 'u', 'c', 't', 'b', 'o', 'a', 'r', 'd', nil]

Parsed row after the fix (correct):

['productboard']