INT-2183 Fix Multicast Regression Issue

Multicast tests are @Ignored because multicast has specific
requirements on the build machine. A regression issue was
introduced by INT-2050.

spring-integration-ip/src/main/java/org/springframework/integration/ip/udp/MulticastSendingMessageHandler.java

@@ -128,7 +128,7 @@ public class MulticastSendingMessageHandler extends UnicastSendingMessageHandler
 			}
 			this.setSocket(socket);
 		}
-		return this.getSocket();
+		return this.getTheSocket();
 	}