Correct code and/or docstrings or both

[pombredanne]

In get_primary_license, the docstring is not correct,
The text says:

Return a license expression string which is the primary license for the
debian copyright file.

but nothing is returned

https://github.com/nexB/scancode-toolkit/blob/1fdf40faabc0cf91fb3e7e8ff11412fcfb358577/src/packagedcode/debian_copyright.py#L374

[adityasangave]

I did try and solve this issue and found out the problem was only with docstring, should I open a PR