Added test for registering when user is already registered.

seasoned_api/test/system/asAUserIWantToGetErrorWhenRegisteringExistingUsername.js

@@ -0,0 +1,20 @@
+const assert = require('assert');
+const request = require('supertest-as-promised');
+const app = require('src/webserver/app');
+const createUser = require('test/helpers/createUser');
+const resetDatabase = require('test/helpers/resetDatabase');
+
+describe('As a user I want error when registering existing username', () => {
+  before(() => {
+    return resetDatabase()
+    .then(() => createUser('test_user', 'password'))
+  })
+
+  it('should return 401 with error message when same username is given', () =>
+    request(app)
+    .post('/api/v1/user')
+    .send({ username: 'test_user', password: 'password' })
+    .expect(401)
+    .then(response => assert.equal(response.text, '{"success":false,"error":"That username is already registered"}'))
+  );
+});