Update XmlSubscriptionStorage.cs

The old code assumes there's a directory in the path, which might not be true. Fixed so it works even without a directory in the path.

Ex: "subscriptions.xml" doesn't work, but ".\subscriptions.xml" works.

Another method could be to throw a meaningful exception if dir is null/empty.
Merge pull request #212 from hagbarddenstore/patch-1

Update XmlSubscriptionStorage.cs