This commit fixes a flaw in the Cask test helper, causing the broken `Cask::Config` test to actually fail.
The flaw occurred while patching the `Cask::Config::DEFAULT_DIRS` hash.
While the original hash uses strings as values, the patched one used
`Pathname` values, masking a broken `Cask::Config::from_json` test.
Now the broken test fails like it should.