Allow fixture_path to be a Pathname

When setting the `fixture_path` using `Rails.root.join('my_fixture/path')` Rails throws an error that it can't find the file `rails_path/my_fixture/path/.yml`. This error is not very informative and takes a long time to track down. By converting the `fixture_path` to a String before getting its size, future users can easily be spared from wasting time tracking this down.