Bugfix: incorrect subscription URI when unsubscribing.

[thales-productive-bjorn]

Unsubscribing without the fix results in a path with 2 times "subscription" in the URI (so "subscription/subscription/type//consumer" whereas it should be "subscription/type//consumer").

This can be verified by running the "client-demo-subscriber" project (from the sos-examples).
When running that example and then quit the subscriber application will result in the following exception:

Exception in thread "Thread-0" eu.arrowhead.common.exception.ArrowheadRuntimeException: eventhandler/subscription/subscription/type/hello/consumer/client-demo-subscriber is not a valid path!
        at eu.arrowhead.common.api.clients.HttpClient.check(HttpClient.java:206)
        at eu.arrowhead.common.api.clients.HttpClient.send(HttpClient.java:152)
        at eu.arrowhead.common.api.clients.OrchestrationStrategy.send(OrchestrationStrategy.java:70)
        at eu.arrowhead.common.api.clients.OrchestrationStrategy$Never.request(OrchestrationStrategy.java:103)
        at eu.arrowhead.common.api.clients.HttpClient.request(HttpClient.java:115)
        at eu.arrowhead.common.api.clients.HttpClient.request(HttpClient.java:95)
        at eu.arrowhead.common.api.clients.core.EventHandlerClient.unsubscribe(EventHandlerClient.java:140)
        at eu.arrowhead.common.api.clients.core.EventHandlerClient.lambda$unsubscribe$0(EventHandlerClient.java:155)
        at java.lang.Iterable.forEach(Iterable.java:75)
        at eu.arrowhead.common.api.clients.core.EventHandlerClient.unsubscribe(EventHandlerClient.java:155)
        at java.util.HashMap.forEach(HashMap.java:1289)
        at eu.arrowhead.common.api.clients.core.EventHandlerClient.unsubscribe(EventHandlerClient.java:163)
        at java.util.HashMap.forEach(HashMap.java:1289)
        at eu.arrowhead.common.api.clients.core.EventHandlerClient.unsubscribeAll(EventHandlerClient.java:171)
        at eu.arrowhead.common.api.ArrowheadApplication.lambda$start$0(ArrowheadApplication.java:172)
        at java.lang.Thread.run(Thread.java:748)