Handle ipv6 addresses in host-header correctly with TLS

When a url uses an ipv6-addresses as the host part, the host-header of the request will be

[::1]:3000

(for ipv6 address ::1). To verify the IP address against a TLS certificate, we need to extract the
IP-address correctly.

Requires node with nodejs/node#14736 resolved to work.

lib/_http_agent.js

@@ -196,13 +196,8 @@ Agent.prototype.addRequest = function addRequest(req, options, port/*legacy*/,
   options = util._extend({}, options);
   options = util._extend(options, this.options);
 
-  if (!options.servername) {
-    options.servername = options.host;
-    const hostHeader = req.getHeader('host');
-    if (hostHeader) {
-      options.servername = hostHeader.replace(/:.*$/, '');
-    }
-  }
+  if (!options.servername)
+    options.servername = calculateServerName(options, req);
 
   var name = this.getName(options);
   if (!this.sockets[name]) {
@@ -258,13 +253,8 @@ Agent.prototype.createSocket = function createSocket(req, options, cb) {
   options = util._extend({}, options);
   options = util._extend(options, self.options);
 
-  if (!options.servername) {
-    options.servername = options.host;
-    const hostHeader = req.getHeader('host');
-    if (hostHeader) {
-      options.servername = hostHeader.replace(/:.*$/, '');
-    }
-  }
+  if (!options.servername)
+    options.servername = calculateServerName(options, req);
 
   var name = self.getName(options);
   options._agentKey = name;
@@ -342,6 +332,29 @@ Agent.prototype.createSocket = function createSocket(req, options, cb) {
   }
 };
 
+function calculateServerName(options, req) {
+  let servername = options.host;
+  const hostHeader = req.getHeader('host');
+  if (hostHeader) {
+    // abc => abc
+    // abc:123 => abc
+    // [::1] => ::1
+    // [::1]:123 => ::1
+    if (hostHeader.startsWith('[')) {
+      const index = hostHeader.indexOf(']');
+      if (index === -1) {
+        // Leading '[', but no ']'. Need to do something...
+        servername = hostHeader;
+      } else {
+        servername = hostHeader.substr(1, index - 1);
+      }
+    } else {
+      servername = hostHeader.split(':', 1)[0];
+    }
+  }
+  return servername;
+}
+
 Agent.prototype.removeSocket = function removeSocket(s, options) {
   var name = this.getName(options);
   debug('removeSocket', name, 'writable:', s.writable);