Update JRMPInvoker to not cast exported object to RemoteStub

If one runs with the jdk5 java.rmi.server.ignoreStubClasses dynamic stub override to force the server to generate stubs for exported objects, the JRMPInvoker fails to start with the following exception:

[starksm@succubus bin]$ run.sh -D-Djava.rmi.server.ignoreStubClasses=true
=========================================================================

...

11:55:52,477 WARN [ServiceController] Problem starting service jboss:service=invoker,type=jrmp
java.lang.ClassCastException: $Proxy13
at org.jboss.invocation.jrmp.server.JRMPInvoker.exportCI(JRMPInvoker.java:437)
at org.jboss.invocation.jrmp.server.JRMPInvoker.startService(JRMPInvoker.java:359)
at org.jboss.invocation.jrmp.server.JRMPInvoker$1.startService(JRMPInvoker.java:136)
at org.jboss.system.ServiceMBeanSupport.jbossInternalStart(ServiceMBeanSupport.java:274)

The reason is that the code creating the remote object is explictly casting the export result to a RemoteStub:

protected void exportCI() throws Exception

{ this.invokerStub = (RemoteStub) UnicastRemoteObject.exportObject (this, rmiPort, clientSocketFactory, serverSocketFactory); }

even though the public contract for this stub only requires a Serializable instance. The type of the invokerStub should just be relaxed to Serializable.