Fix mistaken double backslash

The module helper function run_command was appending a literal backslash
followed by 'n' to the stdin of a command it runs unless you called it
with binary_data=True (not the default).  I have changed it to what
I expect was the intent, to append an actual line feed to stdin.
Merge pull request #6084 from dsedivec/devel

Fix mistaken double backslash