Description of problem:
Using a code like this:
KieServicesConfiguration config = createJmsConfiguration();
config.setMarshallingFormat(MarshallingFormat.JSON);
config.setExtraJaxbClasses(Collections.singleton(clazz));
...
RuleServicesClient rules;
...
rules.executeCommands("testContainer", commands.newBatchExecution(commandList));

Throws unsupported operation exception:
java.lang.UnsupportedOperationException: null
	at java.util.AbstractCollection.add(AbstractCollection.java:262)
	at org.kie.server.api.marshalling.json.JSONMarshaller.<init>(JSONMarshaller.java:69)
	at org.kie.server.api.marshalling.MarshallerFactory.getMarshaller(MarshallerFactory.java:41)
	at org.kie.server.client.impl.AbstractKieServicesClientImpl.<init>(AbstractKieServicesClientImpl.java:72)
	at org.kie.server.client.impl.KieServicesClientImpl.<init>(KieServicesClientImpl.java:60)
	at org.kie.server.client.KieServicesFactory.newKieServicesClient(KieServicesFactory.java:93)
	at com.redhat.xpaas.brms.transport.TransportTest.evaluateRules(TransportTest.java:249)
	at com.redhat.xpaas.brms.transport.TransportTest.testRestJson(TransportTest.java:144)


Version-Release number of selected component (if applicable):
kie-server-api 6.3.0.Final.redhat-1

How reproducible:
Always

Additional info:
The problem is in JSONMarshaller:69 when the passed set is singleton/unmodifiable instance.