Update PJL rspec to comply with guidelines

Basically the updated version is more explicit. If a moethod doesn't return anything but might raise an error, then we focus on that. Also use . to # for instance methods.

Rerun with the latest changes:

```
$ rspec spec/lib/rex/proto/pjl/client_spec.rb 
Rex::Proto::PJL::Client ................

Finished in 0.00867 seconds
16 examples, 0 failures
```