Support element node traversal in document fragments.

https://bugzilla.gnome.org/show_bug.cgi?id=733900
diff --git a/tree.c b/tree.c
index 7d41481..53b956c 100644
--- a/tree.c
+++ b/tree.c
@@ -3456,6 +3456,7 @@
         case XML_ELEMENT_NODE:
         case XML_ENTITY_NODE:
         case XML_DOCUMENT_NODE:
+        case XML_DOCUMENT_FRAG_NODE:
         case XML_HTML_DOCUMENT_NODE:
             cur = parent->children;
             break;
@@ -3491,6 +3492,7 @@
         case XML_ELEMENT_NODE:
         case XML_ENTITY_NODE:
         case XML_DOCUMENT_NODE:
+        case XML_DOCUMENT_FRAG_NODE:
         case XML_HTML_DOCUMENT_NODE:
             cur = parent->children;
             break;
@@ -3526,6 +3528,7 @@
         case XML_ELEMENT_NODE:
         case XML_ENTITY_NODE:
         case XML_DOCUMENT_NODE:
+        case XML_DOCUMENT_FRAG_NODE:
         case XML_HTML_DOCUMENT_NODE:
             cur = parent->last;
             break;