maintain taint flag when encoding MIME on old perl

On perl 5.8.8 and below, loading utf8_heavy.pl (which happens implictly
in many places) will cause a split using a character class to not
maintain the taint flag.  Since the MIME header encoding reconstructs
the string after splitting it, we need to explicitly copy the taint flag
using a substr($s, 0, 0).

lib/Encode/MIME/Header.pm

@@ -139,7 +139,7 @@ sub encode($$;$) {
         push @line, join( "\n " => @subline );
     }
     $_[1] = '' if $chk;
-    return join( "\n", @line );
+    return (substr($str, 0, 0) . join( "\n", @line ));
 }
 
 use constant HEAD   => '=?UTF-8?';