Clear UTF8 flag from HTTP::Message string. Current libwww (5.810) fails on valid
ASCII strings with UTF8 flag with message like "HTTP::Message content not bytes".

--- lib/RPC/XML/Server.pm
+++ lib/RPC/XML/Server.pm
@@ -70,6 +70,7 @@ use vars qw($VERSION @ISA $INSTANCE $INSTALL_DIR @XPL_PATH
 use Carp 'carp';
 use AutoLoader 'AUTOLOAD';
 use File::Spec;
+use Encode;
 
 BEGIN {
     $INSTALL_DIR = (File::Spec->splitpath(__FILE__))[1];
@@ -1641,6 +1642,7 @@ sub process_request
                 # Treat the content strictly in-memory
                 $buf = $respxml->as_string;
                 $buf = Compress::Zlib::compress($buf) if $do_compress;
+                Encode::_utf8_off($buf);
                 $resp->content($buf);
                 $resp->content_length($respxml->length);
             }