title: Firebase cloud messaging does not work after successfully sendToDevice
body: <p>I use Firebase Cloud Functions for sending notification via Cloud Messaging to iOS device. <code>sendToDevice</code> method works great and returns success promise without any error or warning in Firebase Functions logs, but on iOS, notification is not showed. If I send notification from Firebase Notifications dashboard, it works great and notification is showed on the device. I don't know where I can have bug. It's possible that bug is on iOS app side, even when it works like I said?</p>
  
  <p>Firebase configuration:</p>
  
  <pre><code> var functions = require('firebase-functions');
   var admin = require('firebase-admin');
   var config = functions.config();
   admin.initializeApp(functions.config().firebase);
  </code></pre>
  
  <p>Part of cloud function:</p>
  
  <pre><code>APP.services.firebase.admin.database().ref('tokens/' + userId).once('value', function(snapshot) {
  
      var userDevicesTokens = [];
  
      snapshot.forEach(function(childSnapshot) {
          userDevicesTokens.push(childSnapshot.key)
      });
  
      if (userDevicesTokens.length === 0) {
          console.warn('No tokens for user');
          return;
      }
  
      var payload = {
          data: {
              title: options.title,
              text: options.text,
              type: options.type,
          }
      };
      APP.services.firebase.admin.messaging().sendToDevice(userDevicesTokens, payload)
          .then(function(response) {
  
              console.log("Successfully sent message:", response);
  
          })
          .catch(function(error) {
              console.log("Error sending message:", error);
          });
  
  })
  </code></pre>
  
  <p>Firebase Cloud Functions log:</p>
  
  <pre><code>11: 52: 43.952 AM info newChatMessageTrigger
  Successfully sent message: {
      results: [{
          messageId: '0:15016....'
      }],
      canonicalRegistrationTokenCount: 0,
      failureCount: 0,
      successCount: 1,
      multicastId: 589....
  }
  
  11: 52: 22.760 AM outlined_flag newChatMessageTrigger
  Function execution took 604 ms, finished with status: 'ok'
  
  11: 52: 22.252 AM outlined_flag newChatMessageTrigger
  Billing account not configured.External network is not accessible and quotas are severely limited.Configure billing account to remove these restrictions
  
  11: 52: 22.252 AM outlined_flag newChatMessageTrigger
  Function execution started
  </code></pre>