Don't include a port number in the Host header.

We never intend to ask for a service on a port other than the one we
connect to. By my reading of RFC 2616 section 14.23, we can leave the
port number off in this case. Robin Wood reported that
https://twitter.com/ gives you a redirect instead of a page if you send
it
	Host: twitter.com:443
rather than
	Host: twitter.com

http://seclists.org/nmap-dev/2013/q1/267

nselib/http.lua

@@ -147,21 +147,9 @@ local function table_augment(to, from)
 end
 
 --- Get a value suitable for the Host header field.
+-- See RFC 2616 sections 14.23 and 5.2.
 local function get_host_field(host, port)
-  local hostname = stdnse.get_hostname(host)
-  local portno
-  if port == nil then
-    portno = 80
-  elseif type(port) == "table" then
-    portno = port.number
-  else
-    portno = port
-  end
-  if portno == 80 then
-    return hostname
-  else
-    return hostname .. ":" .. tostring(portno)
-  end
+  return stdnse.get_hostname(host)
 end
 
 -- Skip *( SP | HT ) starting at offset. See RFC 2616, section 2.2.