title: Push notification is not working on Background
body: <p>I'm using <a href="http://ionicframework.com/docs/native/firebase/" rel="nofollow noreferrer"><strong>native firebase</strong></a> plugin's push notification feature.</p>
  
  <p><strong>Current behavior:</strong></p>
  
  <p>It is working fine with the foreground.But when the app is in the background and I try to tap the notification it shows <code>undefined</code>.Please see the video below.I have tested this on Anroid device.</p>
  
  <p><strong>Expected behavior:</strong></p>
  
  <p>When I tap the notification message it should open the app and show the message correctly.</p>
  
  <p><strong><a href="https://youtu.be/1lBzwf6grWs" rel="nofollow noreferrer">Video</a></strong></p>
  
  <p><strong>app.component.ts</strong></p>
  
  <pre><code>constructor(public platform: Platform)
  {
      platform.ready().then(() =&gt; {
             this.onNotificationOpen();
        });
  }
  
  onNotificationOpen(): void {
          if (this.platform.is('cordova')) {
              this.firebase.onNotificationOpen()
                  .subscribe(res =&gt; {
                      if (res.tap) {//background mode
                          console.log(res.body);
                          this.showAlert(res.body);
                      } else if (!res.tap) {//foreground mode
                          console.log(res.body);
                          this.showAlert(res.body);
                      }
                  });
          }
      }
  
          showAlert(message: string): void {
          let alert = this.alertCtrl.create({
              title: 'Push Notification',
              subTitle: message,
              buttons: ['OK']
          });
          alert.present();
      }
  </code></pre>
  
  <p><strong>package.json</strong></p>
  
  <pre><code>{
    "name": "ionic-hello-world",
    "author": "Ionic Framework",
    "homepage": "http://ionicframework.com/",
    "private": true,
    "scripts": {
      "clean": "ionic-app-scripts clean",
      "build": "ionic-app-scripts build",
      "ionic:build": "ionic-app-scripts build",
      "ionic:serve": "ionic-app-scripts serve"
    },
    "dependencies": {
      "@angular/common": "4.1.0",
      "@angular/compiler": "4.1.0",
      "@angular/compiler-cli": "4.1.0",
      "@angular/core": "4.1.0",
      "@angular/forms": "4.1.0",
      "@angular/http": "4.1.0",
      "@angular/platform-browser": "4.1.0",
      "@angular/platform-browser-dynamic": "4.1.0",
      "@ionic-native/core": "3.6.1",
      "@ionic-native/device": "3.6.1",
      "@ionic-native/facebook": "^3.10.0",
      "@ionic-native/firebase": "^3.10.3",
      "@ionic-native/keyboard": "^3.10.3",
      "@ionic-native/splash-screen": "3.6.1",
      "@ionic-native/status-bar": "3.6.1",
      "@ionic/cli-plugin-cordova": "^1.2.0",
      "@ionic/cli-plugin-ionic-angular": "^1.2.0",
      "@ionic/storage": "2.0.1",
      "@ngrx/core": "^1.2.0",
      "@ngrx/store": "^2.2.1",
      "d3": "^4.7.4",
      "firebase": "^3.7.5",
      "ionic-angular": "3.2.1",
      "ionicons": "3.0.0",
      "moment": "^2.18.1",
      "pouchdb": "^6.1.2",
      "pouchdb-adapter-cordova-sqlite": "^2.0.2",
      "rxjs": "5.1.1",
      "sw-toolbox": "3.6.0",
      "zone.js": "0.8.10"
    },
    "devDependencies": {
      "@ionic/app-scripts": "1.3.7",
      "@ionic/cli-plugin-cordova": "1.2.1",
      "@ionic/cli-plugin-ionic-angular": "1.2.0",
      "typescript": "~2.2.1"
    },
    "cordovaPlugins": [
      "cordova-plugin-whitelist",
      "cordova-plugin-console",
      "cordova-plugin-statusbar",
      "cordova-plugin-device",
      "cordova-plugin-splashscreen",
      "ionic-plugin-keyboard"
    ],
    "cordovaPlatforms": [
      "ios",
      {
        "platform": "ios",
        "version": "",
        "locator": "ios"
      }
    ],
    "description": "helloWorld: An Ionic project"
  }
  </code></pre>