[C] use picker creation date as default Date (#944)

* [C] use picker creation date as default Date

* fix test

Xamarin.Forms.Core.UnitTests/DatePickerUnitTest.cs

@@ -162,7 +162,7 @@ namespace Xamarin.Forms.Core.UnitTests
 		{
 			var datePicker = new DatePicker ();
 			Assert.DoesNotThrow (() => datePicker.SetValue (DatePicker.DateProperty, null));
-			Assert.AreEqual (DatePicker.DateProperty.DefaultValue, datePicker.Date);
+			Assert.AreEqual (DateTime.Today, datePicker.Date);
 		}
 
 		[Test]

Xamarin.Forms.Core/DatePicker.cs

@@ -8,8 +8,10 @@ namespace Xamarin.Forms
 	{
 		public static readonly BindableProperty FormatProperty = BindableProperty.Create(nameof(Format), typeof(string), typeof(DatePicker), "d");
 
-		public static readonly BindableProperty DateProperty = BindableProperty.Create(nameof(Date), typeof(DateTime), typeof(DatePicker), DateTime.Today, BindingMode.TwoWay, coerceValue: CoerceDate,
-			propertyChanged: DatePropertyChanged);
+		public static readonly BindableProperty DateProperty = BindableProperty.Create(nameof(Date), typeof(DateTime), typeof(DatePicker), default(DateTime), BindingMode.TwoWay,
+			coerceValue: CoerceDate,
+			propertyChanged: DatePropertyChanged,
+			defaultValueCreator: (bindable) => DateTime.Today);
 
 		public static readonly BindableProperty MinimumDateProperty = BindableProperty.Create(nameof(MinimumDate), typeof(DateTime), typeof(DatePicker), new DateTime(1900, 1, 1),
 			validateValue: ValidateMinimumDate, coerceValue: CoerceMinimumDate);