BUG: Calling ElectrumCluster.subscribe() and then ElectrumCluster.unsubscribe() throws error

(Might have some relation to #73 (closed) )

When using an ElectrumCluster, calling a ElectrumCluster.subscribe(...) and then calling the corresponding ElectrumCluster.unsubscribe(...) with the same parameters, it will fail with:

Error: Cannot unsubscribe from 'blockchain.transaction.subscribe' since it has no subscription with the given parameters.

NOTE: This only affects ElectrumCluster and not ElectrumClient.