NodeJS Mock HTTP Post Request for Integration Testing

I'm building a front-end application that makes a request to another web service and get's a response on a POST request:

http://greetings_api:3000/greeting
  data {'language': 'es'}
Response: 'Hola'

I'm using nock to override the HTTP response:

module.exports = {
    test: function(req, res) {
      var http = require('http');
      var nock = require('nock');
      var greeting;

      nock('http://greetings_api:3000').
        post('/greeting').
        socketDelay(2000).
        reply(200, 'Hola');

      var options = {
        hostname: 'greetings_api',
        port: 3000,
        path: '/greeting',
        method: 'POST',
        headers: {
            'Content-Type': 'application/json',
            'language': 'es'
        }
      };

      var req = http.request(options, function(res) {
        res.setEncoding('utf8');
        res.on('data', function (body) {
          greeting = body;
        });
      });
      req.on('error', function(e) {
        console.log('problem with request: ' + e.message);
      });

      return res.send(greeting + ' tester')
    }
};

My problem is that my tests are failing because greeting is being set properly:

1) Test Route
     Test() function
       Should welcome us:
   AssertionError: expected 'undefined tester' to include 'Hola tester'
    at Context.<anonymous> (/tests/routes/app.test.js:20:43)

Why is greeting not being properly defined here and/or why is the request not being intercepted properly?