fixed xmlCopyDoc to also copy the doc->URL as pointed by Martijn Faassen

* tree.c: fixed xmlCopyDoc to also copy the doc->URL as pointed
  by Martijn Faassen
Daniel
diff --git a/tree.c b/tree.c
index 31511c5..8ef2fc8 100644
--- a/tree.c
+++ b/tree.c
@@ -4164,6 +4164,8 @@
         ret->name = xmlMemStrdup(doc->name);
     if (doc->encoding != NULL)
         ret->encoding = xmlStrdup(doc->encoding);
+    if (doc->URL != NULL)
+        ret->URL = xmlStrdup(doc->URL);
     ret->charset = doc->charset;
     ret->compression = doc->compression;
     ret->standalone = doc->standalone;