Fix binary output bug related to read_data_line.

This bug was introduced when `read_data_line` was refactored in PR
"Clean up I/O" \#358 where the function interface changed from `type`
to `data_type` and no longer used strings to describe the return type.
Merge pull request #362 from mandli/binary-read-data-bug-fix

Fix binary output bug related to read_data_line.