Respond to keep-alive with response flag off. (#194)

#### Problem

`ServerReactiveSocket` upon receiving a `KeepAlive` frame responds with a `KeepAlive` frame as an ack. This ack MUST not have respond flag set to true. Doing so will result in an infinite loop of keep-alive frames sent between the peers.

#### Modification

`ServerReactiveSocket` responds with the respond flag turned off.

#### Result

No infinite loop of keep-alive frames.

Author: NiteshKant

reactivesocket-core/src/main/java/io/reactivesocket/ServerReactiveSocket.java

@@ -327,7 +327,7 @@ private Publisher<Void> handleFireAndForget(int streamId, Publisher<Void> result
 
     private Publisher<Void> handleKeepAliveFrame(Frame frame) {
         if (Frame.Keepalive.hasRespondFlag(frame)) {
-            return Px.from(connection.sendOne(frame))
+            return Px.from(connection.sendOne(Frame.Keepalive.from(Frame.NULL_BYTEBUFFER, false)))
                 .doOnError(errorConsumer);
         }
         return Px.empty();

reactivesocket-core/src/test/java/io/reactivesocket/ServerReactiveSocketTest.java

@@ -43,7 +43,8 @@ public void testHandleKeepAlive() throws Exception {
         rule.connection.addToReceivedBuffer(Frame.Keepalive.from(Frame.NULL_BYTEBUFFER, true));
         Frame sent = rule.connection.awaitSend();
         assertThat("Unexpected frame sent.", sent.getType(), is(FrameType.KEEPALIVE));
-        assertThat("Unexpected keep-alive frame respond flag.", Frame.Keepalive.hasRespondFlag(sent), is(true));
+        /*Keep alive ack must not have respond flag else, it will result in infinite ping-pong of keep alive frames.*/
+        assertThat("Unexpected keep-alive frame respond flag.", Frame.Keepalive.hasRespondFlag(sent), is(false));
     }