Subscription tests and automatically coerce subscription id to string

[armstrys]

Adding a test that makes sure content in the subscription class is consistent with expected types when converted to json and back. Notably, the current class doesn't force the id to be a string.

I've also removed the default filters value of None since it is currently unused in the body of the function and isn't a valid option for a subscription - maybe it makes sense to replace this with an empty Filters object instead?