Change the behaviour of generated getter and setter

Perhaps a matter of "taste".
I don't like passing 'nil' to have the getter behaviour.
It seams like I'm setting the banner to "nil".
In most code we see just the method (without arguments).

So, instead of

```ruby
@parser.banner nil
```

just

```ruby
@parser.banner
```

and it will do the job.

But, if this is not your taste, feel free to drop unmerged this pull request.

(Perhaps there's a better way to do it).