Do not copy None, Inherit and NotSet at DeepCopy (#565)

* Do not copy None, Inherit and NotSet at DeepCopy.
* Add test.

Source/Painting/SvgColourServer.cs

@@ -73,6 +73,9 @@ namespace Svg
 
         public override SvgElement DeepCopy<T>()
         {
+            if (this == None || this == Inherit || this == NotSet)
+                return this;
+
             var newObj = base.DeepCopy<T>() as SvgColourServer;
             newObj.Colour = this.Colour;
             return newObj;

Tests/Svg.UnitTests/SvgElementDeepCopyTest.cs

@@ -71,6 +71,14 @@ namespace Svg.UnitTests
             Assert.IsInstanceOf<T>(dest);
         }
 
+        [Test]
+        public void TestDoNotDeepCopy()
+        {
+            Assert.AreSame(SvgPaintServer.None, SvgPaintServer.None.DeepCopy());
+            Assert.AreSame(SvgColourServer.Inherit, SvgColourServer.Inherit.DeepCopy());
+            Assert.AreSame(SvgColourServer.NotSet, SvgColourServer.NotSet.DeepCopy());
+        }
+
         /// <summary>
         /// Tests that the deep copy of a <see cref="SvgText"/> is done correctly where the
         /// text element has contains only text and now other elements like <see cref="SvgTextSpan"/>.