xpath BUGFIX copy root_type to all sets

Fixes #970
diff --git a/src/xpath.c b/src/xpath.c
index 72e3aa0..f616c1c 100644
--- a/src/xpath.c
+++ b/src/xpath.c
@@ -744,6 +744,7 @@
     if (set) {
         new->ctx = set->ctx;
         new->ctx_node = set->ctx_node;
+        new->root_type = set->root_type;
         new->local_mod = set->local_mod;
         new->trees = set->trees;
         new->format = set->format;