Default the SERVER_PORT to 80.

- because mongrel2 does not seem to pass the port as part of the "host"
  in the headers when its 80 (the default HTTP port).

lib/Plack/Handler/Mongrel2.pm

@@ -82,6 +82,7 @@ sub mongrel2_req_to_psgi {
     }
     $env{SERVER_PROTOCOL} = delete $hdrs->{VERSION};
     ($env{SERVER_NAME}, $env{SERVER_PORT}) = split /:/, delete $hdrs->{host}, 2;
+    $env{SERVER_PORT} ||= 80;
 
     foreach my $key (keys %$hdrs) {
         my $new_key = uc $key;
@@ -389,4 +390,4 @@ This library is available under Artistic License v2, and is:
 
 Daisuke Maki C<< <daisuke@endeworks.jp> >>
 
-=cut
+=cut