Added equality test.

And fix bug in `DashStyle`.

src/Avalonia.Visuals/Media/DashStyle.cs

@@ -10,7 +10,7 @@ namespace Avalonia.Media
         {
             get
             {
-                if (dashDotDot == null)
+                if (dash == null)
                 {
                     dash = new DashStyle(new double[] { 2, 2 }, 1);
                 }

tests/Avalonia.Visuals.UnitTests/Media/PenTests.cs

@@ -1,4 +1,5 @@
 using Avalonia.Media;
+using Avalonia.Media.Immutable;
 using Xunit;
 
 namespace Avalonia.Visuals.UnitTests.Media
@@ -29,5 +30,27 @@ namespace Avalonia.Visuals.UnitTests.Media
 
             Assert.True(raised);
         }
+
+        [Fact]
+        public void Equality_Is_Implemented_Between_Mutable_And_Immutable_Pens()
+        {
+            var brush = new SolidColorBrush(Colors.Red);
+            var target1 = new Pen(
+                brush: brush,
+                thickness: 2,
+                dashStyle: DashStyle.Dash,
+                lineCap: PenLineCap.Round,
+                lineJoin: PenLineJoin.Round,
+                miterLimit: 21);
+            var target2 = new ImmutablePen(
+                brush: brush,
+                thickness: 2,
+                dashStyle: DashStyle.Dash,
+                lineCap: PenLineCap.Round,
+                lineJoin: PenLineJoin.Round,
+                miterLimit: 21);
+
+            Assert.True(Equals(target1, target2));
+        }
     }
 }