test: test Equals(object) for various Drawing types

This brings coverage to 100% for the affected types.

X10D.Tests/src/Drawing/CuboidTests.cs

@@ -40,6 +40,12 @@ public class CuboidTests
         Assert.IsTrue(Cuboid.Cube != Cuboid.Empty);
     }
 
+    [TestMethod]
+    public void Equals_ShouldBeFalse_GivenDifferentObjects()
+    {
+        Assert.IsFalse(Cuboid.Cube.Equals(null));
+    }
+
     [TestMethod]
     public void GetHashCode_ShouldBeCorrect_GivenEmptyCircle()
     {

X10D.Tests/src/Drawing/EllipseFTests.cs

@@ -60,6 +60,12 @@ public class EllipseFTests
         Assert.IsTrue(EllipseF.Unit != EllipseF.Empty);
     }
 
+    [TestMethod]
+    public void Equals_ShouldBeFalse_GivenDifferentObjects()
+    {
+        Assert.IsFalse(EllipseF.Unit.Equals(null));
+    }
+
     [TestMethod]
     public void GetHashCode_ShouldBeCorrect_GivenEmptyEllipse()
     {

X10D.Tests/src/Drawing/EllipseTests.cs

@@ -51,6 +51,12 @@ public class EllipseTests
         Assert.IsTrue(Ellipse.Unit != Ellipse.Empty);
     }
 
+    [TestMethod]
+    public void Equals_ShouldBeFalse_GivenDifferentObjects()
+    {
+        Assert.IsFalse(Ellipse.Unit.Equals(null));
+    }
+
     [TestMethod]
     public void GetHashCode_ShouldBeCorrect_GivenEmptyEllipse()
     {

X10D.Tests/src/Drawing/Line3DTests.cs

@@ -88,6 +88,12 @@ public class Line3DTests
         Assert.IsTrue(Line3D.One != Line3D.Empty);
     }
 
+    [TestMethod]
+    public void Equals_ShouldBeFalse_GivenDifferentObjects()
+    {
+        Assert.IsFalse(Line3D.One.Equals(null));
+    }
+
     [TestMethod]
     public void GetHashCode_ShouldBeCorrect_GivenEmptyLine()
     {

X10D.Tests/src/Drawing/LineFTests.cs

@@ -80,6 +80,12 @@ public class LineFTests
         Assert.IsTrue(LineF.One != LineF.Empty);
     }
 
+    [TestMethod]
+    public void Equals_ShouldBeFalse_GivenDifferentObjects()
+    {
+        Assert.IsFalse(LineF.One.Equals(null));
+    }
+
     [TestMethod]
     public void GetHashCode_ShouldBeCorrect_GivenEmptyLine()
     {

X10D.Tests/src/Drawing/LineTests.cs

@@ -62,6 +62,12 @@ public class LineTests
         Assert.IsTrue(Line.One != Line.Empty);
     }
 
+    [TestMethod]
+    public void Equals_ShouldBeFalse_GivenDifferentObjects()
+    {
+        Assert.IsFalse(Line.One.Equals(null));
+    }
+
     [TestMethod]
     public void GetHashCode_ShouldBeCorrect_GivenEmptyLine()
     {

X10D.Tests/src/Drawing/SphereTests.cs

@@ -66,6 +66,12 @@ public class SphereTests
         Assert.IsFalse(unitCircle1 != unitCircle2);
     }
 
+    [TestMethod]
+    public void Equals_ShouldBeFalse_GivenDifferentObjects()
+    {
+        Assert.IsFalse(Sphere.Unit.Equals(null));
+    }
+
     [TestMethod]
     public void Equals_ShouldBeFalse_GivenDifferentCircles()
     {