bugfix attempt for https://github.com/wq/django-natural-keys/issues/8

[marcosox]

This should fix #8 by running the function only when its argument is a string (and thus a natural key slug)

[marcosox]

Please note I am not a maintainer of this project, I just found this library and just started to use it.

I updated the patch. I'd like to know your opinion on it, since I'm not that experienced as a python programmer.

[marcosox]

I don't know why travis is running the tests against the old commit (c8aecc6) instead of the new one (2ab32b4). I run them locally and they all pass.

[marcosox]

@sheppard do you have any idea about why travis is complaining in #10 and #11 ? Can you merge the pull requests?

[sheppard]

Thanks, I finally merged this. After rebasing and fixing conflicts with #9 (and #11), the main effect of this merge is the str check. There is a small chance that a valid slug might be something other than a str (such as a Python2 unicode instance). But we can wait to see if that even comes up.