title: error in angular2firebase snapshotchanges() subscription
body: <p>I get a changes.forEach error when trying to get data from firebase using snapshotChanges() but it works fine if i use valuechanges(). I'm not sure what i'm doing wrong so pls help</p>
  
  <pre><code>import { Injectable } from '@angular/core';
  import { AngularFirestore, AngularFirestoreCollection, 
  AngularFirestoreDocument } from "angularfire2/firestore";
  import { BehaviorSubject } from "rxjs/BehaviorSubject";
  import { of } from "rxjs/Observable/of";
  import {Observable} from 'rxjs';
  import 'rxjs/add/operator/map';
  import { Employee } from "../models/employee";`
  
  @Injectable()
  export class EmployeeService {
  employees: Observable&lt;Employee[]&gt;;
  employeeCollection: AngularFirestoreCollection;`
  
  constructor( public db: AngularFirestore ) {
  db.firestore.settings({ timestampsInSnapshots: true });
  this.employeeCollection = this.db.collection('employee');
  this.employees = this.employeeCollection.snapshotChanges().map(changes =&gt; {
   return changes.map(a =&gt; {
     const data = a.payload.doc.data() as Employee;
      data.id = a.payload.doc.id;
      return data;
      });
   });
  }
  
  getEmployees(){
    return this.employees;
  }
  </code></pre>
  
  <p>and this is my component</p>
  
  <pre><code>import { Component, OnInit } from '@angular/core';
  import { EmployeeService } from "../../services/employee.service";
  import { Employee } from "../../models/employee";
  
  @component({
    selector: 'app-employees',
    templateUrl: './employees.component.html',
    styleUrls: ['./employees.component.css']
    })
  
  export class EmployeesComponent implements OnInit {
  employees: Employee[];
  constructor(
     private employeeService: EmployeeService
  ) { }`
  
  ngOnInit() {
   this.employeeService.getEmployees().subscribe(employees =&gt; {
   this.employees = employees;
  });`
  </code></pre>
  
  <p>then I get this error on my chrome console</p>
  
  <p><img src="https://i.stack.imgur.com/0kzQD.png" alt="chrome"></p>