Add default parameter value of zero to INavigationPageController.Peek() (#708)

Xamarin.Forms.Core.UnitTests/NavigationUnitTest.cs

@@ -248,6 +248,7 @@ namespace Xamarin.Forms.Core.UnitTests
 			await nav.PushAsync(child2);
 
 			Assert.AreEqual(((INavigationPageController)nav).Peek(0), child2);
+			Assert.AreEqual(((INavigationPageController)nav).Peek(), child2);
 		}
 
 		[Test]

Xamarin.Forms.Core/INavigationPageController.cs

@@ -7,7 +7,7 @@ namespace Xamarin.Forms
 {
 	public interface INavigationPageController
 	{
-		Page Peek(int depth);
+		Page Peek(int depth = 0);
 
 		IEnumerable<Page> Pages { get; }