[core] Get stream-open-like element from transport

When sending a stream-open-like element, it depends on the actual used
transport which element is send. For example, RFC6120-style TCP uses
<stream>, whereas the Websocket binding for XMPP uses <open/>.

smack-core/src/main/java/org/jivesoftware/smack/AbstractXMPPConnection.java

@@ -2229,7 +2229,7 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
         }
     }
 
-    protected void sendStreamOpen() throws NotConnectedException, InterruptedException {
+    protected final void sendStreamOpen() throws NotConnectedException, InterruptedException {
         // If possible, provide the receiving entity of the stream open tag, i.e. the server, as much information as
         // possible. The 'to' attribute is *always* available. The 'from' attribute if set by the user and no external
         // mechanism is used to determine the local entity (user). And the 'id' attribute is available after the first
@@ -2241,12 +2241,17 @@ public abstract class AbstractXMPPConnection implements XMPPConnection {
             from = XmppStringUtils.completeJidFrom(localpart, to);
         }
         String id = getStreamId();
+        String lang = config.getXmlLang();
 
-        StreamOpen streamOpen = new StreamOpen(to, from, id, config.getXmlLang(), StreamOpen.StreamContentNamespace.client);
+        AbstractStreamOpen streamOpen = getStreamOpen(to, from, id, lang);
         sendNonza(streamOpen);
         updateOutgoingStreamXmlEnvironmentOnStreamOpen(streamOpen);
     }
 
+    protected AbstractStreamOpen getStreamOpen(CharSequence to, CharSequence from, String id, String lang) {
+        return new StreamOpen(to, from, id, lang);
+    }
+
     protected void updateOutgoingStreamXmlEnvironmentOnStreamOpen(AbstractStreamOpen streamOpen) {
         XmlEnvironment.Builder xmlEnvironmentBuilder = XmlEnvironment.builder();
         xmlEnvironmentBuilder.with(streamOpen);

smack-core/src/main/java/org/jivesoftware/smack/c2s/ModularXmppClientToServerConnection.java

@@ -560,6 +560,12 @@ public final class ModularXmppClientToServerConnection extends AbstractXMPPConne
         waitForConditionOrThrowConnectionException(() -> featuresReceived, waitFor);
     }
 
+    @Override
+    protected AbstractStreamOpen getStreamOpen(CharSequence to, CharSequence from, String id, String lang) {
+        StreamOpenAndCloseFactory streamOpenAndCloseFactory = activeTransport.getStreamOpenAndCloseFactory();
+        return streamOpenAndCloseFactory.createStreamOpen(to, from, id, lang);
+    }
+
     protected void newStreamOpenWaitForFeaturesSequence(String waitFor) throws InterruptedException,
                     SmackException, XMPPException {
         prepareToWaitForFeaturesReceived();

smack-core/src/main/java/org/jivesoftware/smack/packet/StreamOpen.java

@@ -1,6 +1,6 @@
 /**
  *
- * Copyright © 2014-2019 Florian Schmaus
+ * Copyright © 2014-2020 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,6 +33,10 @@ public final class StreamOpen extends AbstractStreamOpen {
         this(to, from, id, "en", StreamContentNamespace.client);
     }
 
+    public StreamOpen(CharSequence to, CharSequence from, String id, String lang) {
+        super(to, from, id, lang);
+    }
+
     public StreamOpen(CharSequence to, CharSequence from, String id, String lang, StreamContentNamespace ns) {
         super(to, from, id, lang, ns);
     }