Make ContentPresenter update its DataContext.

Previous method introduced in 4241fd8331
caused issue #503, because ContentPresenter should only use Content as
its DataContext when it is not a control. Take the WPF approach and make
ContentPresenter choose its own DataContext. Fixes #503.

src/Perspex.Controls/Presenters/ContentPresenter.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
 using System;
+using System.Reactive.Linq;
 using Perspex.Controls.Primitives;
 using Perspex.Controls.Templates;
 using Perspex.Layout;
@@ -83,6 +84,17 @@ namespace Perspex.Controls.Presenters
             TemplatedParentProperty.Changed.AddClassHandler<ContentPresenter>(x => x.TemplatedParentChanged);
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="ContentPresenter"/> class.
+        /// </summary>
+        public ContentPresenter()
+        {
+            var dataContext = this.GetObservable(ContentProperty)
+                .Select(x => x is IControl ? PerspexProperty.UnsetValue : x);
+
+            Bind(Control.DataContextProperty, dataContext);
+        }
+
         /// <summary>
         /// Gets or sets a brush with which to paint the background.
         /// </summary>

tests/Perspex.Controls.UnitTests/Presenters/ContentPresenterTests.cs

@@ -130,6 +130,28 @@ namespace Perspex.Controls.UnitTests.Presenters
             Assert.IsType<Decorator>(target.Child);
         }
 
+        [Fact]
+        public void Assigning_Control_To_Content_Should_Not_Set_DataContext()
+        {
+            var target = new ContentPresenter
+            {
+                Content = new Border(),
+            };
+
+            Assert.False(target.IsSet(Control.DataContextProperty));
+        }
+
+        [Fact]
+        public void Assigning_NonControl_To_Content_Should_Set_DataContext()
+        {
+            var target = new ContentPresenter
+            {
+                Content = "foo",
+            };
+
+            Assert.Equal("foo", target.DataContext);
+        }
+
         private class TestContentControl : TemplatedControl
         {
             public IControl Child { get; set; }