http: refactor to make servername option normalization testable

Refs: https://coverage.nodejs.org/coverage-36bb31be5f0b85a0/lib/_http_agent.js.html#L316
PR-URL: nodejs#38733
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

lib/_http_agent.js

@@ -253,8 +253,7 @@ Agent.prototype.addRequest = function addRequest(req, options, port/* legacy */,
   if (options.socketPath)
     options.path = options.socketPath;
 
-  if (!options.servername && options.servername !== '')
-    options.servername = calculateServerName(options, req);
+  normalizeServerName(options, req);
 
   const name = this.getName(options);
   if (!this.sockets[name]) {
@@ -313,8 +312,7 @@ Agent.prototype.createSocket = function createSocket(req, options, cb) {
   if (options.socketPath)
     options.path = options.socketPath;
 
-  if (!options.servername && options.servername !== '')
-    options.servername = calculateServerName(options, req);
+  normalizeServerName(options, req);
 
   const name = this.getName(options);
   options._agentKey = name;
@@ -344,6 +342,11 @@ Agent.prototype.createSocket = function createSocket(req, options, cb) {
     oncreate(null, newSocket);
 };
 
+function normalizeServerName(options, req) {
+  if (!options.servername && options.servername !== '')
+    options.servername = calculateServerName(options, req);
+}
+
 function calculateServerName(options, req) {
   let servername = options.host;
   const hostHeader = req.getHeader('host');