Fixed validating "required" on non-scalar values

Validating with the "required" rule fails when the field value is non-scalar, eg an object or array. The bug seems to be caused by passing the field value through trim() without first checking if the value may be cast to a string.

Recreating the error:
```php
$gump = new GUMP();
$gump->validation_rules([
    "name"    => "required",
    "address" => "required"
]);

// The "address" field fails validation because the field value is an object.
$values = [
    "name"    => "headzoo",
    "address" => new stdClass()
];
$is_valid = $gump->run($values);
```
Merge pull request #26 from headzoo/patch-1

Fixed validating "required" on non-scalar values