fix unsupported operation exception when adding to an unmodifiable set

data-store/src/main/java/org/lucares/pdb/datastore/internal/DataStore.java

@@ -190,7 +190,7 @@ public class DataStore implements AutoCloseable {
 		final Long oldDocId = tagsToDocId.putValue(tags, docId);
 		Preconditions.checkNull(oldDocId, "There must be at most one document for tags: {0}", tags);
 
-		final SortedSet<Tag> ts = tags.toTags();
+		final List<Tag> ts = new ArrayList<>(tags.toTags());
 		ts.add(TAG_ALL_DOCS);
 		for (final Tag tag : ts) {