Ionic 3 native POST sending no data

I'm using express as my back-end and I want to simply post JSON to it. I can get the request to the server but when it arrives it's empty and I can't find my data... Here is my code:

import {Injectable} from "@angular/core";
import { HTTP } from '@ionic-native/http';
import {User} from "./user";
import {Config} from "../config";
import {Observable} from "rxjs/Rx";
import 'rxjs/add/operator/map';

@Injectable()
export class UserService {

  constructor(private _http: HTTP) {}


  //Makes http request to valid username and password
  login(user: User) {
    //Creates header to tell server that it is getting json

    //Sets address to request
    var address = Config.apiUrl + "login/";

    //Creates JSON object that contains the data
    let data = JSON.stringify({
      username: user.username,
      password: user.password
    });

    console.log(data);
    //Make request to server
    return this._http.post(address, data, {"Content-Type":"application/json"})
    .then(res => {
      return(res);
    })
    .catch(error => {
      return Observable.throw(error);
    })
  }
}

Thank you! I'd also like to note that I've seen other people say that changing the header to x-www-form-urlencoded will fix it, but I want to send my information with JSON as I plan on using JSON web tokens in the future.