Fix filter with a specified source

Hi,

When we create a filter like that
```
fail2ban::filter { 'apache-badbots':
        filtersource => 'puppet:///modules/modules/fail2ban/apache-badbots.conf',
}
fail2ban::jail { 'apache-bodbots': }
```

The file resulting filter File contain both ```content``` and ```source``` with throw an exception.

This pull request correct this behavior.

Regards