Fix for issue #6

Using fix suggested by sorora in issue #6 thread as it worked for me. NOT authored by myself, pulled from this commit here https://github.com/sorora/Laravel-Test-Helpers/commit/4658c228959f03ca46a4c8c5bdfdfb2f34fe65b4 reduced switch to 2 cases as relationship method only uses 2 arguments ($relation, $foreignKey) correct?
Also 1st time pull request so apologies for doing anything wrong :P