Fixed file store to handle delete_matched being called before cache dir is created.

[phuibonhoa]

Added fix so that file store does not raise an exception when cache dir does not exist yet. This can happen if a delete_matched is called before anything is saved in the cache.

[josevalim]

Thanks for the pull request. Could we improve the test here? The current test is calling a private method but i believe we should be testing the real behavior that causes the failure. For example, the test should probably 1) create a cache store 2) cache something in it 3) call delete_matched to remove the directory 4) try to lookup for something and ensure an exception won't be raised. Otherwise, someone can refactor the code later, realize that the coupled test to the method does not makes sense anymore while the bug still exists.

[phuibonhoa]

Thanks for the input, you're right that wasn't the best testing approach. I've updated it to test the case where the exception normally is raised (delete_matched is called before anything is ever stored in the cache, so the containing directories are not yet created).