CSHARP-674: Handle null values in the KeyValuePairSerializer.

Added a unit test for a KeyValuePair with a null value.

Author: dhgoldman

MongoDB.BsonUnitTests/Serialization/Serializers/KeyValuePairSerializerTests.cs

@@ -0,0 +1,46 @@
+/* Copyright 2010-2012 10gen Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using NUnit.Framework;
+
+using MongoDB.Bson;
+using MongoDB.Bson.IO;
+using MongoDB.Bson.Serialization;
+using MongoDB.Bson.Serialization.Attributes;
+
+namespace MongoDB.BsonUnitTests.DefaultSerializer.Serializers
+{
+    [TestFixture]
+    public class KeyValuePairSerializerTests
+    {
+        [Test]
+        public void TestNullValue()
+        {
+            KeyValuePair<string, object> kvp = new KeyValuePair<string, object>("Value", null);
+            var json = kvp.ToJson();
+            var expected = "{ 'k' : 'Value', 'v' : null }".Replace("'", "\"");
+            Assert.AreEqual(expected, json);
+
+            var bson = kvp.ToBson();
+            var rehydrated = BsonSerializer.Deserialize<KeyValuePair<string, object>>(bson);
+            Assert.IsTrue(bson.SequenceEqual(rehydrated.ToBson()));
+        }
+    }
+}