Remove duplicated condition branch

Because the #output_header checks if the result has the header, so
it's caller dosen't have to check it.
Don't show strange code by removing condition

"Unless @has_header" specifies "if header exists in CSV, don't output
it.", so if this condition is removed, this code is strange.
Use more meaningfull name

str -> version_string
Merge remote branch 'yoshihara/remove-duplicated-condition-branch'
Merge remote branch 'yoshihara/use-more-meaningful-name'