Minor bug in get_messages() in flexi_auth_lite_model.php

Hi

There is a minor issue in the status_message() an error_message(). Even if the prefix_delimiter and suffix_delimiter are set to false, the library wraps it with the delimiters in the config file.

To fix it on line 577 of get_messages()
```php
if (! prefix_delimiter)
```
needs to be
```php
if ($prefix_delimiter!=FALSE)
```
and the same for if (! $suffix_delimiter)