Q:

Can not get POST parameters when sending POST request with Retrofit

I am using Retrofit 2.0.0 to send POST request to my REST API. 
private class AuthTask extends AsyncTask<String, String, String> {
    protected String doInBackground(String... params) {
        Call<Auth> call = service.auth(params[0], params[1]);
        try {
            Auth authResponse = call.execute().body();
            Log.i(AuthActivity.class.getName(), authResponse.public_key);
        } catch (IOException ex) {
            Log.e(AuthActivity.class.getName(), "Error."); 
            return "error";
        }
        return "success";
    }

    protected void onPostExecute(String result) {
        Log.i(AuthTask.class.getName(), "Done");
    }
}

public class Auth {
    public String public_key;
}

public interface AuthService {
    @FormUrlEncoded
    @POST("auth")
    Call<Auth> auth(@Field("username") String username, @Field("publickey") String publickey);
}

Here is my log output for the request:
12-14 19:36:54.014 30318-30373/com.app D/OkHttp: --> POST /auth HTTP/1.1
12-14 19:36:54.014 30318-30373/com.app D/OkHttp: username=ghtbznz&publickey=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCedhOhP9MgYufDxPKH1DdtJiBBdBtbsUy8R7kg%0AWm1Edm7c3ThToMxMgjvOAsHn8rP1Ka1eVN34hC4HV2%2BoomRaWH25WbunN9ZVRqBnowsTOd40eEKh%0A6dsO8Cl3u65VsArDyZyEQa7Ofx29i2juOpWRG%2F6tp9FVnJzZt5dBDkWOKwIDAQAB%0A
12-14 19:36:54.014 30318-30373/com.app D/OkHttp: --> END POST (256-byte body)

And here is the log output for the response:
12-14 19:42:20.358 2549-2804/com.app D/OkHttp: <-- HTTP/1.1 200 OK (1790ms)
12-14 19:42:20.358 2549-2804/com.app D/OkHttp: Date: Mon, 14 Dec 2015 18:42:16 GMT
12-14 19:42:20.358 2549-2804/com.app D/OkHttp: Server: Apache/2.2.22 (Debian)
12-14 19:42:20.358 2549-2804/com.app D/OkHttp: Vary: Accept-Encoding
12-14 19:42:20.358 2549-2804/com.app D/OkHttp: Keep-Alive: timeout=5, max=99
12-14 19:42:20.366 2549-2804/com.app D/OkHttp: Connection: Keep-Alive
12-14 19:42:20.366 2549-2804/com.app D/OkHttp: Content-Type: text/html
12-14 19:42:20.366 2549-2804/com.app D/OkHttp: OkHttp-Selected-Protocol: http/1.1
12-14 19:42:20.366 2549-2804/com.app D/OkHttp: OkHttp-Sent-Millis: 1450118540088
12-14 19:42:20.366 2549-2804/com.app D/OkHttp: OkHttp-Received-Millis: 1450118540358
12-14 19:42:20.373 2549-2804/com.app D/OkHttp:  
12-14 19:42:20.373 2549-2804/com.app D/OkHttp:  
12-14 19:42:20.373 2549-2804/com.app D/OkHttp:  
12-14 19:42:20.373 2549-2804/com.app D/OkHttp: array(0) {
12-14 19:42:20.373 2549-2804/com.app D/OkHttp: }
12-14 19:42:20.373 2549-2804/com.app D/OkHttp: Notice: Undefined index: username in /home/auth/index.php on line 45
12-14 19:42:20.373 2549-2804/com.app D/OkHttp: Notice: Undefined index: publickey in /home/auth/index.php on line 46
12-14 19:42:20.373 2549-2804/com.app D/OkHttp: <-- END HTTP (251-byte body)

In my PHP-file, this is how I retrieve the POST parameters:
$user = $_POST["username"];
$pk = $_POST["publickey"];

The strange thing is, when I use CURL to send the POST request everything works.  So my question is why am I not able to send the proper POST parameters with Retrofit? 

A:

I installed the application Shark on my Android phone and sniffed the traffic.  Finally, I found out that a redirect occurs (301) and therefore, all the POST parameters are gone. 
The solution is very simple.  Just add a slash after the REST endpoint:
public interface AuthService {
    @FormUrlEncoded
    @POST("auth/")
    Call<Auth> auth(@Field("username") String username, @Field("publickey") String publickey);
}

