MDL-83538 mod_lti: explicitly set content-type header in token endpoint

Prevents a fallback to 'text/html', as set in setup.php.

mod/lti/token.php

@@ -111,6 +111,7 @@
 EOD;
 }
 
+$response->set_content_type('application/json');
 $response->set_body($body);
 
-$response->send();
+$response->send();