Keep prev data only when delta negotiated (#78)

Author: FZambia

centrifuge/build.gradle

@@ -69,3 +69,11 @@ javadoc {
 metalava {
     hiddenPackages.add("io.github.centrifugal.centrifuge.internal")
 }
+
+test {
+    testLogging {
+        events "passed", "skipped", "failed"
+        exceptionFormat "full"
+        showStandardStreams = true
+    }
+}

centrifuge/src/main/java/io/github/centrifugal/centrifuge/Subscription.java

@@ -179,11 +179,13 @@ void handlePublication(Protocol.Publication pub) throws Exception {
         ClientInfo info = ClientInfo.fromProtocolClientInfo(pub.getInfo());
         PublicationEvent event = new PublicationEvent();
         byte[] pubData = pub.getData().toByteArray();
-        byte[] prevData = this.getPrevData();
-        if (prevData != null && pub.getDelta()) {
-            pubData = Fossil.applyDelta(prevData, pubData);
+        if (this.deltaNegotiated) {
+            byte[] prevData = this.getPrevData();
+            if (prevData != null && pub.getDelta()) {
+                pubData = Fossil.applyDelta(prevData, pubData);
+            }
+            this.setPrevData(pubData);
         }
-        this.setPrevData(pubData);
         event.setData(pubData);
         event.setInfo(info);
         event.setOffset(pub.getOffset());