When mod_auth_mellon sends logout request to the idP, it gets response back
through HTTP GET request. That works fine, However, some idP implementations
send the logout response through POST request. Logout response via POST is not
understood by mod_auth_mellon and leads to 404 page. The mod_auth_mellon
assumes that POST method in logout endpoint is always SOAP logout request. The
suggested fix adds a check and POST handler for logout responses.