message: Fix: Map user._id to ObjectId
diff: import {promisify} from 'util';
  import {Response} from 'express';
  import {
  - NotFoundError, ForbiddenError, ContentType, UseBefore, Param, Res, Controller,
  - Body, Post, Get, Delete, CurrentUser, Authorized
  + Authorized,
  + Body,
  + ContentType,
  + Controller,
  + CurrentUser,
  + Delete,
  + ForbiddenError,
  + Get,
  + NotFoundError,
  + Param,
  + Post,
  + Res,
  + UseBefore
  } from 'routing-controllers';
  import passportJwtMiddleware from '../security/passportJwtMiddleware';
  import {Unit} from '../models/units/Unit';
  @@ -16,12 +27,12 @@ import {errorCodes} from '../config/errorCodes';
  import * as fs from 'fs';
  import * as path from 'path';
  +import {File} from '../models/mediaManager/File';
  +import {ICourse} from '../../../shared/models/ICourse';
  +import * as mongoose from 'mongoose';
  const archiver = require('archiver');
  import crypto = require('crypto');
  -import {User} from '../models/User';
  -import {File} from '../models/mediaManager/File';
  -import {ICourse} from '../../../shared/models/ICourse';
  const cache = require('node-file-cache').create({life: config.timeToLiveCacheValue});
  const pdf = require('html-pdf');
  @@ -130,7 +141,7 @@ export class DownloadController {
  }
  async createFileHash(courseName: string, userId: string) {
  - let data = [
  + const data = [
  courseName, userId, Date.now()
  ];
  @@ -459,7 +470,7 @@ export class DownloadController {
  return true;
  }
  - if (course.courseAdmin._id === user._id) {
  + if (mongoose.Types.ObjectId(user._id).equals(course.courseAdmin._id)) {
  return true;
  }
  