Special case string -> bool conversion.

src/Markup/Perspex.Markup/Binding/LogicalNotNode.cs

@@ -23,14 +23,28 @@ namespace Perspex.Markup.Binding
         {
             if (v != PerspexProperty.UnsetValue)
             {
-                try
+                var s = v as string;
+
+                if (s != null)
                 {
-                    var boolean = Convert.ToBoolean(v, CultureInfo.InvariantCulture);
+                    bool result;
-                    return !boolean;
+
+                    if (bool.TryParse(s, out result))
+                    {
+                        return !result;
+                    }
                 }
-                catch
+                else
                 {
-                    // TODO: Maybe should log something here.
+                    try
+                    {
+                        var boolean = Convert.ToBoolean(v, CultureInfo.InvariantCulture);
+                        return !boolean;
+                    }
+                    catch
+                    {
+                        // TODO: Maybe should log something here.
+                    }
                 }
             }
 

tests/Perspex.Markup.UnitTests/Binding/ExpressionObserverTests_Negation.cs

@@ -61,7 +61,7 @@ namespace Perspex.Markup.UnitTests.Binding
         }
 
         [Fact]
-        public async void Should_Return_Empty_For_String_Not_Convertible_To_Boolean()
+        public async void Should_Return_UnsetValue_For_String_Not_Convertible_To_Boolean()
         {
             var data = new { Foo = "foo" };
             var target = new ExpressionObserver(data, "!Foo");