Wrap ConfigOrigin.comments in unmodifiableList

This prevents people from messing with the returned list.

config/src/main/java/com/typesafe/config/impl/SimpleConfigOrigin.java

@@ -221,7 +221,7 @@ final class SimpleConfigOrigin implements ConfigOrigin {
     @Override
     public List<String> comments() {
         if (commentsOrNull != null) {
-            return commentsOrNull;
+            return Collections.unmodifiableList(commentsOrNull);
         } else {
             return Collections.emptyList();
         }