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.
CSHARP-603: GSSAPI on linux using libgsasl.
CSHARP-603: update for changes to driver authentication spec.
removed CRAM-MD5 and DIGEST-MD5 support.
corrected issue with SSPI utilizing cached credentials.  It is now required to specify the username for GSSAPI credentials.
fixing unit tests.
initial removal of MongoAuthenticationMechanism enum
addressing change made in server 2.4rc0.
fixed bug with Code property and added tests.