Fix incorrect use of sock.get that can lead to an indefinite timeout

console1:
```
msf> use auxiliary/scanner/http/open_proxy
msf auxiliary(open_proxy) > set RHOSTS 192.168.0.4
msf auxiliary(open_proxy) > set RPORT 8888
msf auxiliary(open_proxy) > run
< the connection never times out >
```

console2:
```
$ nc -vlp 8888
Listening on [0.0.0.0] (family 0, port 8888)
Connection from [192.168.0.4] port 8888 [tcp/*] accepted (family 2, sport 43245)
GET http://209.85.148.147/ HTTP/1.1
Host: 209.85.148.147
Connection: close
User-Agent: user_agent
Accept-Encoding: *
Accept-Charset: ISO-8859-1,UTF-8;q=0.7,*;q=0.7
Cache-Control: no
Accept-Language: de,en;q=0.7,en-us;q=0.3
```

After the patch, requests timeout after 10 seconds:
```
msf auxiliary(open_proxy) > run
[*] Scanned 1 of 1 hosts (100% complete)
[*] Auxiliary module execution completed
```

modules/auxiliary/scanner/http/open_proxy.rb

@@ -115,7 +115,7 @@ class Metasploit3 < Msf::Auxiliary
 
       request = write_request('GET',site,user_agent)
       sock.put(request)
-      res = sock.get
+      res = sock.get_once(-1, 10)
 
       disconnect