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

@@ -1283,11 +1283,13 @@ Agent.prototype.createSocket = function(name, host, port, localAddress, req) {
   options.host = host;
   options.localAddress = localAddress;
 
-  options.servername = host;
-  if (req) {
-    var hostHeader = req.getHeader('host');
-    if (hostHeader) {
-      options.servername = hostHeader.replace(/:.*$/, '');
+  if (!options.servername) {
+    options.servername = host;
+    if (req) {
+      var hostHeader = req.getHeader('host');
+      if (hostHeader) {
+        options.servername = hostHeader.replace(/:.*$/, '');
+      }
     }
   }