title: Retrofit enqueue doesn't work but execute work
body: <p>I'm using <strong>com.squareup.retrofit2:retrofit:2.2.0</strong></p>
  
  <pre><code>    HttpLoggingInterceptor logging = new HttpLoggingInterceptor();
      logging.setLevel(HttpLoggingInterceptor.Level.BODY);
  
      OkHttpClient.Builder httpClient = new OkHttpClient.Builder();
      httpClient.addInterceptor(logging);
  
  
      Retrofit retrofit = new Retrofit.Builder()
              .baseUrl("http://localhost:9000")
              .addConverterFactory(GsonConverterFactory.create())
              .client(httpClient.build())
              .build();
  
      final UserService service = retrofit.create(UserService.class);
  
      Call&lt;User&gt; userCall = service.createUser(user)
  </code></pre>
  
  <p>Here is the problem: when I run the <code>execute</code> it make REST API request but when I use <code>enqueue</code> it does nothing (no exception, no log)</p>
  
  <pre><code>  userCall.execute(); //this work
  
    //this does not work
    userCall.enqueue(new Callback&lt;User&gt;() {
        @Override
        public void onResponse(Call&lt;User&gt; call, Response&lt;User&gt; response) {
            // no response 
        }
  
        @Override
        public void onFailure(Call&lt;User&gt; call, Throwable t) {
            //nothing here
        }
    });
  </code></pre>