Update MongoDB.Bson/Serialization/Serializers/KeyValuePairSerializer.cs

If the value of a generic KeyValuePair is null, use the Object serializer.
CSHARP-674: Handle null values in the KeyValuePairSerializer.

Added a unit test for a KeyValuePair with a null value.
CSHARP-674: Handle null keys as well as null values in KeyValuePairSerializer.