Description of problem:
If you call Collection<ProcessInstance> processInstances = ksession.getProcessInstances();
when using the rest endpoints you end up with the following exception : 

Exception in thread "main" java.lang.ClassCastException: org.kie.services.client.serialization.jaxb.impl.task.JaxbTaskResponse cannot be cast to java.util.Collection
	at org.drools.core.command.impl.CommandBasedStatefulKnowledgeSession.getProcessInstances(CommandBasedStatefulKnowledgeSession.java:135)
	at org.jbpm.simple.rest.client.SimpleRestClient.main(SimpleRestClient.java:44)




Version-Release number of selected component (if applicable):


How reproducible:
Clone the jbpm playground (https://github.com/droolsjbpm/jbpm-playground/tree/master/jbpm-simple-rest-client) repository and open the jbpm-simple-rest-client

Add to the example the following lines:
        Collection<ProcessInstance> processInstances = ksession.getProcessInstances();
        for(ProcessInstance pi : processInstances){
            System.out.println("Process Instance: "+pi.getProcessId() + pi.getProcessName());
        }

Notice that if you don't try to use the process instance collection you will not get an error. Also notice that in order to reproduce this issue you  need to get running the KIE-WB. Due the fact that unit tests doesn't marshall all the data as a real client.

Steps to Reproduce:
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Actual results:


Expected results:


Additional info: