Adding EditCell constructor validation (#301)

* New unit tests!
* New CopyTo method!
* New null validation!

YEAH!!!

test/Microsoft.SqlTools.ServiceLayer.UnitTests/QueryExecution/DbCellValueTests.cs

@@ -0,0 +1,53 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using Microsoft.SqlTools.ServiceLayer.QueryExecution.Contracts;
+using Xunit;
+
+namespace Microsoft.SqlTools.ServiceLayer.UnitTests.QueryExecution
+{
+    public class DbCellValueTests
+    {
+        [Fact]
+        public void ConstructValid()
+        {
+            // If: I construct a new DbCellValue
+            DbCellValue dbc = new DbCellValue
+            {
+                DisplayValue = "qqq",
+                IsNull = true,
+                RawObject = 12
+            };
+
+            // Then: It should have the values I specified in it
+            Assert.Equal("qqq", dbc.DisplayValue);
+            Assert.Equal(12, dbc.RawObject);
+            Assert.True(dbc.IsNull);
+        }
+
+        [Fact]
+        public void CopyToNullOther()
+        {
+            // If: I copy a DbCellValue to null
+            // Then: I should get an exception
+            Assert.Throws<ArgumentNullException>(() => new DbCellValue().CopyTo(null));
+        }
+
+        [Fact]
+        public void CopyToValid()
+        {
+            // If: I copy a DbCellValue to another DbCellValue
+            DbCellValue source = new DbCellValue {DisplayValue = "qqq", IsNull = true, RawObject = 12};
+            DbCellValue dest = new DbCellValue();
+            source.CopyTo(dest);
+
+            // Then: The source values should be in the dest
+            Assert.Equal(source.DisplayValue, dest.DisplayValue);
+            Assert.Equal(source.IsNull, dest.IsNull);
+            Assert.Equal(source.RawObject, dest.RawObject);
+        }
+    }
+}