MDL-9399 auth/ldap: NTLM redirect only on initial GET

We only want to redirect when users are being shown the login page (on
GET) but not when users are POSTing to it, actually trying to login.

auth/ldap/auth.php

@@ -1745,7 +1745,9 @@ function change_password_url() {
     function loginpage_hook() {
         global $CFG;
 
-        if (!empty($this->config->ntlmsso_enabled)  // SSO enabled
+        if ($_SERVER['REQUEST_METHOD'] === 'GET'    // Only on initial GET 
+                                                    // of loginpage
+            &&!empty($this->config->ntlmsso_enabled)// SSO enabled
             && !empty($this->config->ntlmsso_subnet)// have a subnet to test for
             && empty($_GET['authldap_skipntlmsso']) // haven't failed it yet
             && (isguestuser() || !isloggedin())     // guestuser or not-logged-in users