fix the issue "Trying to get property of...

... non-object in ./plagiarism/turnitin/lib.php on line 335/1246
$plagiarismsettings should be an associative array not an object and everywhere else in the code you use it as an array
line 284:        
if (empty($plagiarismsettings['use_turnitin'])) {
line 559:       
if (!empty($plagiarismsettings['turnitin_assignid'])) {