mogfiledebug: show Last-Modified header if available

This can be useful in several cases:

1) it may be used to determine the original version of the file
   if one copy was corrupted during replication

2) it can also be used to determine when replication congestion
   occurred by comparing the relative differences between
   replicas.

mogfiledebug

@@ -126,6 +126,7 @@ if (@paths) {
         }
         print " - MD5 Hash: ", $res->{hash}, "\n";
         print " - Length: ", $res->{length}, "\n" if defined $res->{length};
+        print " - Last-Modified: ", $res->{mtime}, "\n" if defined $res->{mtime};
         print " - HTTP result: ", $res->{res}, "\n";
     }
 } else {
@@ -183,6 +184,7 @@ sub fetch_path {
 
     $toret{hash}  = $ctx->hexdigest;
     $toret{res}   = $res->status_line;
+    $toret{mtime} = $res->header("Last-Modified");
     $toret{path}  = $path;
     return \%toret;
 }