Fix PerspexProperty.IsSet.

src/Perspex.Base/PerspexObject.cs

@@ -457,8 +457,15 @@ namespace Perspex
         public bool IsSet(PerspexProperty property)
         {
             Contract.Requires<ArgumentNullException>(property != null);
+            
+            PriorityValue value;
+
+            if (_values.TryGetValue(property, out value))
+            {
+                return value.Value != PerspexProperty.UnsetValue;
+            }
 
-            return _values.ContainsKey(property);
+            return false;
         }
 
         /// <summary>

tests/Perspex.Base.UnitTests/PerspexObjectTests_Metadata.cs

@@ -42,6 +42,35 @@ namespace Perspex.Base.UnitTests
             Assert.Equal(new[] { "Attached" }, names);
         }
 
+        [Fact]
+        public void IsSet_Returns_False_For_Unset_Property()
+        {
+            var target = new Class1();
+
+            Assert.False(target.IsSet(Class1.FooProperty));
+        }
+
+        [Fact]
+        public void IsSet_Returns_False_For_Set_Property()
+        {
+            var target = new Class1();
+
+            target.SetValue(Class1.FooProperty, "foo");
+
+            Assert.True(target.IsSet(Class1.FooProperty));
+        }
+
+        [Fact]
+        public void IsSet_Returns_False_For_Cleared_Property()
+        {
+            var target = new Class1();
+
+            target.SetValue(Class1.FooProperty, "foo");
+            target.SetValue(Class1.FooProperty, PerspexProperty.UnsetValue);
+
+            Assert.False(target.IsSet(Class1.FooProperty));
+        }
+
         private class Class1 : PerspexObject
         {
             public static readonly PerspexProperty<string> FooProperty =