Add minimal specs for rex/proto/http/packet/header

Verification
----------

- [ ] Run rspec for the new specs and ensure there aren't failures

```
$ rspec spec/lib/rex/proto/http/packet*
Rex::Proto::Http::Packet ....
Rex::Proto::Http::Packet::Header ........

Finished in 0.02264 seconds
12 examples, 0 failures

Randomized with seed 15792
```

- [ ] Ensure travis is green before landing