Make Job::toString handle non-primitive parameters.

Make Job::toString handle non-primitive parameters.

Depending on PHP version, configuration and platform, Job::toString
would cause warnings like this if non-primitive parameters where
used in the job:

Notice: Array to string conversion in C:\xampp\htdocs\wikidata-client\includes\job\Job.php on line 245

This change takes care of providing a sane representation of
non-primitive values.

Change-Id: I00ac52d40d39d8219626f5116172950f1b7af381