Fix bug in xmlrpc + $cfg->usersuniquebyusername

Bug 1556692: When used together, these can cause problems when
the ID field from Moodle gets truncated to the default
get_new_username() length of "30", when being inserted into
usr.username in Mahara.

behatnotneeded: Can't test Mnet in Behat

Change-Id: Icdeb78b5298e7d63a0610987b0d8fad34e58d036