title: OnBackButtonPressed not fired in ViewModel
body: <p>I have a created a simple contentpage to test various events. When overriding OnBackButtonPressed in the ViewModel the event is not raised. According to xamarin the event will not be raised on iOS, but it should work on Android and WP. But I cant get it to work on these platforms either.</p>
  
  <p><strong>What am I missing?</strong></p>
  
  <p><strong>TestPage.xaml</strong></p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt;
  &lt;ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
               xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
               x:Class="TestPage.TestPage"
               Title="testPage"&gt;
  &lt;/ContentPage&gt;
  </code></pre>
  
  <p><strong>TestPage.xaml.cs</strong></p>
  
  <pre><code>public partial class TestPage : ContentPage
  {
      public TestPage ()
      {
          InitializeComponent();
          this.BindingContext = new TestPageViewModel();
      }
  }
  </code></pre>
  
  <p><strong>ViewModel</strong></p>
  
  <pre><code>public class TestPageViewModel : ContentPage, INotifyPropertyChanged
  {
       public TestPageViewModel() {   }
       protected override bool OnBackButtonPressed()
       {
           // Do stuff
           return base.OnBackButtonPressed();
       }
  }
  </code></pre>