cleanup for dlink_dir_300_615_http_login

Hi @m-1-k-3,

This pull request references https://github.com/rapid7/metasploit-framework/pull/1651

Im adding a last cleanup, just fingerprinting dlink device (with server header) before starting the brute forcing. In this way we avoid brute forcing no dlink devices when launching it against a large set of hosts (scanner mode :))

Feel free to review, check, test, discuss anything, and land once you feel comfortable and we'll ready to go I guess (please share pcap of this working as usual :))