Add rspec for PJL client

Hi William,

So I see that you're having a hard time with writing the rspec for Rex::Proto::PJL::Client. I understand we all kind of suck at teaching each other how to do this because we're all pretty new to it, but here's my rpsec for your code with 100% coverage. It also serves an an example for you and hopefully this can improve your rspec skills.

Verification step:
- [ ] Run the rspec like this: ```rspec spec/lib/rex/proto/pjl/client_spec.rb```
- [ ] Confirm that all 15 examples pass the test like the following example:

```
$ rspec spec/lib/rex/proto/pjl/client_spec.rb 
Rex::Proto::PJL::Client ...............

Finished in 1.22 seconds
15 examples, 0 failures
```