Messages to room participants are forwarded to the IRC user

For example, both JID #chan%server@biboumi/Toto and toto!server@biboumi are
equivalent, except that if you send a message to the first one, subsequent
messages coming from the user toto will come from that same JID. This is
done to be consistent for the XMPP user, and respond from the same JID than
the 'to' of the first message.

fix #2468

src/xmpp/xmpp_component.hpp

@@ -111,9 +111,13 @@ public:
   void close_document();
   /**
    * Send a message from from@served_hostname, with the given body
+   *
+   * If fulljid is false, the provided 'from' doesn't contain the
+   * server-part of the JID and must be added.
    */
   void send_message(const std::string& from, Xmpp::body&& body,
-                    const std::string& to, const std::string& type);
+                    const std::string& to, const std::string& type,
+                    const bool fulljid=false);
   /**
    * Send a join from a new participant
    */