Allow to reset X-Label from mutt 1.6.x onwards.
Patch provided by Klaus Ethgen <Klaus@Ethgen.de>

Debian bug: http://bugs.debian.org/832971

--- a/copy.c
+++ b/copy.c
@@ -460,8 +460,7 @@
       char *tmp = NULL;
       int fail = 0;
 
-      if (fail == 0 &&
-          ((h->env->kwtypes & MUTT_X_LABEL) || (h->env->kwtypes == 0)) &&
+      if (((h->env->kwtypes & MUTT_X_LABEL) || (h->env->kwtypes == 0)) &&
           (option(OPTKEYWORDSLEGACY) || option(OPTKEYWORDSSTANDARD) == 0))
       {
         mutt_labels(buf, sizeof(buf), h->env, XlabelDelim);