https: don't overwrite servername option

PR-URL: https://github.com/joyent/node/pull/9368
Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>

lib/http.js

@@ -1283,6 +1283,7 @@ Agent.prototype.createSocket = function(name, host, port, localAddress, req) {
   options.host = host;
   options.localAddress = localAddress;
 
+  if (!options.servername) {
     options.servername = host;
     if (req) {
       var hostHeader = req.getHeader('host');
@@ -1290,6 +1291,7 @@ Agent.prototype.createSocket = function(name, host, port, localAddress, req) {
         options.servername = hostHeader.replace(/:.*$/, '');
       }
     }
+  }
 
   var s = self.createConnection(options);
   if (!self.sockets[name]) {