-Added DBNull support to JValue

Src/Newtonsoft.Json.Tests/Linq/JObjectTests.cs

@@ -1394,5 +1394,23 @@ Parameter name: arrayIndex")]
 
       Assert.AreEqual("Name2", value);
     }
+
+    [Test]
+    public void WriteObjectNullDBNullValue()
+    {
+      DBNull dbNull = DBNull.Value;
+      JValue v = new JValue(dbNull);
+      Assert.AreEqual(DBNull.Value, v.Value);
+      Assert.AreEqual(JTokenType.Null, v.Type);
+
+      JObject o = new JObject();
+      o["title"] = v;
+
+      string output = o.ToString();
+      
+      Assert.AreEqual(@"{
+  ""title"": null
+}", output);
+    }
   }
 }

Src/Newtonsoft.Json.Tests/Linq/JValueTests.cs

@@ -56,6 +56,10 @@ namespace Newtonsoft.Json.Tests.Linq
       v.Value = "Pie";
       Assert.AreEqual("Pie", v.Value);
       Assert.AreEqual(JTokenType.String, v.Type);
+
+      v.Value = DBNull.Value;
+      Assert.AreEqual(DBNull.Value, v.Value);
+      Assert.AreEqual(JTokenType.Null, v.Type);
     }
 
     [Test]

Src/Newtonsoft.Json/Linq/JValue.cs

@@ -203,6 +203,8 @@ namespace Newtonsoft.Json.Linq
     {
       if (value == null)
         return JTokenType.Null;
+      else if (value == DBNull.Value)
+        return JTokenType.Null;
       else if (value is string)
         return GetStringValueType(current);
       else if (value is long || value is int || value is short || value is sbyte