Modified AuthController->attemptRegister() validation rules

[rodrigokestler]

Modified AuthController->attemptRegister() validation rules
from:
$rules = array_merge($users->getValidationRules(['only' => ['username']]), [
'email' => 'required|valid_email|is_unique[users.email]',
'password' => 'required|strong_password',
'pass_confirm' => 'required|matches[password]',
]);
to:
$rules = [
'username' => 'required|alpha_numeric_space|min_length[3]|is_unique[users.username]',
'email' => 'required|valid_email|is_unique[users.email]',
'password' => 'required|strong_password',
'pass_confirm' => 'required|matches[password]',
];
because one of the validation rules for username returned by getValidationRules() method
is 'is_unique[users.username,id,{id}'. Specifying the optional 2nd and 3rd parameters will
make the validation library try to ignore the {id} value in the id column in the users table.
Since it is a registration process, the user still does not exist in the DB and its DB ID
can not be used to be ignored in the is_unique validation rule, therefore, throwing a DB error.