use the hashmap

ttnhabbridge/src/main/java/nl/sikken/bertrik/hab/Sentence.java

@@ -64,9 +64,9 @@ public final class Sentence {
         // format basic string
         StringBuilder sb = new StringBuilder();
         sb.append(String.format(Locale.ROOT, "%s,%d,%s", callSign, id, timeString));
-        for (String s : fields) {
+        for (Entry<String, String> s : fields) {
             sb.append(',');
-            sb.append(s);
+            sb.append(s.getValue());
         }
         String basic = sb.toString();