implemented xmlXPathObjectCopy for external objects added


	* xpath.c: implemented xmlXPathObjectCopy for external objects
	* include/libxml/xpathInternals.h: added xmlXPathStackIsExternal
diff --git a/xpath.c b/xpath.c
index 95a516e..64502ee 100644
--- a/xpath.c
+++ b/xpath.c
@@ -2776,8 +2776,10 @@
 	    break;
 	}
 #endif
-	case XPATH_UNDEFINED:
-	case XPATH_USERS:
+        case XPATH_USERS:
+	    ret->user = val->user;
+	    break; 
+        case XPATH_UNDEFINED:
 	    xmlGenericError(xmlGenericErrorContext,
 		    "xmlXPathObjectCopy: unsupported type %d\n",
 		    val->type);