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.
Added Daniel Goldman to the list of contributors in the README.
CSHARP-666: Initial implementation of lazy and raw BsonDocument and BsonArray.
CSHARP-671: server sometimes neglects to return setName causing the driver to think that the instance is not a replica set member and we remove it from memory preventing recovery.