1.3.4: Fix expired subscription handling

Fixed

• Return nil from read_subscription if subscription data has gone from Redis, That was resulted in a "No query string was present" errors received on graphql subscription clients receiving a hash with an nil query_string instead of receiving nil directly. [@thlacroix] (#51)

• Subscription events are no longer dropped when another subscription with the same fingerprint expires during execution. [@prog-supdex] (#53)

• Deleting a subscription no longer goes through the deprecated GraphQL::AnyCable.redis; this avoids both the deprecation warning and reusing a connection after it has been returned to the pool. [@Envek] (#53)

Changed

• GraphQL::Subscriptions::AnyCableSubscriptions#read_subscription now raises GraphQL::AnyCable::SubscriptionExpiredError when the subscription is no longer stored in Redis, instead of returning nil. [@Envek] (#53)