Omit an initial dash of header property names

As I mentioned in the mailing list (http://old.nabble.com/Release-CGI%3A%3AApplication%3A%3APlugin%3A%3AHeader-to35216388.html),
header_props() doesn't bahave as we expect in the following situation:

  my $app = CGI::Application->new;
  $app->header_props( type => 'text/plain' );
  $app->run; # => "Status: text/plain\n..." (broken!)

I hope an initial dash of header prop. name can be omitted.