Set host header instead of host in url

Setting the host in the url might result in the request going
to a completely different server (wherever the hostname resolves
to), not necessarily the server under test.

lib/agent.js

@@ -53,6 +53,9 @@ methods.forEach(function(method) {
     req.ca(this._ca);
     req.cert(this._cert);
     req.key(this._key);
+    if (this._host) {
+      req.set('host', this._host);
+    }
 
     req.on('response', this._saveCookies.bind(this));
     req.on('redirect', this._saveCookies.bind(this));

lib/test.js

@@ -25,15 +25,15 @@ module.exports = Test;
  * @api public
  */
 
-function Test(app, method, path, host) {
+function Test(app, method, path) {
   Request.call(this, method.toUpperCase(), path);
   this.redirects(0);
   this.buffer();
   this.app = app;
   this._asserts = [];
   this.url = typeof app === 'string'
     ? app + path
-    : this.serverAddress(app, path, host);
+    : this.serverAddress(app, path);
 }
 
 /**
@@ -51,15 +51,15 @@ Object.setPrototypeOf(Test.prototype, Request.prototype);
  * @api private
  */
 
-Test.prototype.serverAddress = function(app, path, host) {
+Test.prototype.serverAddress = function(app, path) {
   var addr = app.address();
   var port;
   var protocol;
 
   if (!addr) this._server = app.listen(0);
   port = app.address().port;
   protocol = app instanceof https.Server ? 'https' : 'http';
-  return protocol + '://' + (host || '127.0.0.1') + ':' + port + path;
+  return protocol + '://127.0.0.1:' + port + path;
 };
 
 /**

test/supertest.js

@@ -866,14 +866,15 @@ describe('agent.host(host)', function () {
     const agent = request.agent(app);
 
     app.get('/', function (req, res) {
-      res.send();
+      res.send({ hostname: req.hostname });
     });
 
     agent
       .host('something.test')
       .get('/')
       .end(function (err, res) {
-        err.hostname.should.equal('something.test');
+        if (err) return done(err);
+        res.body.hostname.should.equal('something.test');
         done();
       });
   });