Final cleanup for rfcode_reader_enum

Hi @juushya,

Verified your pcap's here is my last clean up before we're ready to merge. Feel free to check, test, and discuss, ask, etc about anything you doubt about my changes. Once changes look good to you, please, merge this pull request into your repo and the original pull request https://github.com/rapid7/metasploit-framework/pull/1923 will be automatically updated and ready to go. Thanks!

* fake test after changes:

```
msf auxiliary(rfcode_reader_enum) > run

[*] 192.168.172.192:80 - Checking if authentication is required...
[*] 192.168.172.192:80 - Brute-forcing...
[+] 192.168.172.192:80 - SUCCESSFUL LOGIN - "":""
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed

```