Make NetworkConnection send(T data) method synchronized

protocol/src/main/java/ch/zhaw/pm2/multichat/protocol/NetworkHandler.java

@@ -305,7 +305,7 @@ public class NetworkHandler {
          * @param data  data object of type T to be submitted through the connection.
          * @throws IOException if an error occurs (e.g. connection interrupted while sending, ...)
          */
-        public void send(T data) throws IOException {
+        public synchronized void send(T data) throws IOException {
             ObjectOutputStream outputStream = new ObjectOutputStream(socket.getOutputStream());
             outputStream.writeObject(data);
         }