fix handling of custom params and add `oauth_callback`, fixes #1040

Closes #1040

Merge request studip/studip!611

app/controllers/course/lti.php

@@ -242,7 +242,7 @@ class Course_LtiController extends StudipController
         ]);
 
         foreach ($custom_parameters as $param) {
-            list($key, $value) = explode('=', $param);
+            list($key, $value) = explode('=', $param, 2);
             if (isset($value)) {
                 $lti_link->addCustomParameter(trim($key), trim($value));
             }
@@ -351,7 +351,7 @@ class Course_LtiController extends StudipController
         ]);
 
         foreach ($custom_parameters as $param) {
-            list($key, $value) = explode('=', $param);
+            list($key, $value) = explode('=', $param, 2);
             if (isset($value)) {
                 $lti_link->addCustomParameter(trim($key), trim($value));
             }

lib/classes/LtiLink.php

@@ -51,6 +51,7 @@ class LtiLink
             'lti_version' => 'LTI-1p0',
             'lti_message_type' => 'basic-lti-launch-request',
             'oauth_consumer_key' => $this->consumer_key,
+            'oauth_callback' => 'about:blank',
             'oauth_version' => '1.0',
             'oauth_nonce' => uniqid('lti', true),
             'oauth_timestamp' => time(),