Fix sap ni proxy, hopefully

It's related to https://github.com/rapid7/metasploit-framework/pull/1018, it's my try to fix the sap ni proxy code and hopefully we'll be able to go with this awesome capability. Please feel free to check, review, test, comment discuss anything, and once ready, merge it into your branch and the original https://github.com/rapid7/metasploit-framework/pull/1018 will be automatically updated!

My test after modification:

```
msf > use auxiliary/scanner/http/http_version 
msf auxiliary(http_version) > set Proxies sapni:192.168.172.179:3299
Proxies => sapni:192.168.172.179:3299
msf auxiliary(http_version) > set RHOSTS 192.168.172.216
RHOSTS => 192.168.172.216
msf auxiliary(http_version) > run

[*] 192.168.172.216:80 Apache/2.2.14 (Ubuntu)
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed

```