Add empty data to SSH_MSG_IGNORE messages (#974)

As required by RFC 4253 Section 11.2, and RFC 4251 Section 5

Co-authored-by: Jeroen van Erp <jeroen@hierynomus.com>

src/itest/java/com/hierynomus/sshj/transport/kex/StrictKeyExchangeTest.java

@@ -145,7 +145,7 @@ class StrictKeyExchangeTest {
 
         @Override
         protected void doKeepAlive() throws TransportException {
-            conn.getTransport().write(new SSHPacket(Message.IGNORE));
+            conn.getTransport().write(new SSHPacket(Message.IGNORE).putString(""));
         }
 
     }

src/main/java/net/schmizz/keepalive/Heartbeater.java

@@ -29,6 +29,6 @@ final class Heartbeater
 
     @Override
     protected void doKeepAlive() throws TransportException {
-        conn.getTransport().write(new SSHPacket(Message.IGNORE));
+        conn.getTransport().write(new SSHPacket(Message.IGNORE).putString(""));
     }
 }

src/test/java/net/schmizz/keepalive/HeartbeaterTest.java

@@ -0,0 +1,51 @@
+/*
+ * Copyright (C)2009 - SSHJ Contributors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package net.schmizz.keepalive;
+
+import net.schmizz.sshj.DefaultConfig;
+import net.schmizz.sshj.common.Message;
+import net.schmizz.sshj.common.SSHPacket;
+import net.schmizz.sshj.connection.ConnectionImpl;
+import net.schmizz.sshj.transport.Transport;
+
+import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentCaptor;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+class HeartbeaterTest {
+
+  @Test
+  void ignoreMessageContainsData() throws Exception {
+    Transport transport = mock(Transport.class);
+    when(transport.getConfig()).thenReturn(new DefaultConfig());
+    ArgumentCaptor<SSHPacket> sshPacketCaptor = ArgumentCaptor.forClass(SSHPacket.class);
+    when(transport.write(sshPacketCaptor.capture())).thenReturn(0L);
+    ConnectionImpl connection = new ConnectionImpl(transport, KeepAliveProvider.HEARTBEAT);
+
+    KeepAlive heartbeater = connection.getKeepAlive();
+    assertThat(heartbeater).isInstanceOf(Heartbeater.class);
+
+    heartbeater.doKeepAlive();
+
+    SSHPacket sshPacket = sshPacketCaptor.getValue();
+    assertThat(sshPacket.readMessageID()).isEqualTo(Message.IGNORE);
+    assertThat(sshPacket.readBytes()).isNotNull();
+  }
+
+}