Return DBNull.Value instead of null in SQL value encoding methods

Better fix for issue #286

MimeKit/Cryptography/X509CertificateDatabase.cs

@@ -228,9 +228,9 @@ AsymmetricKeyParameter DecodePrivateKey (DbDataReader reader, int column, ref by
 			return DecryptAsymmetricKeyParameter (buffer, nread);
 		}
 
-		byte[] EncodePrivateKey (AsymmetricKeyParameter key)
+		object EncodePrivateKey (AsymmetricKeyParameter key)
 		{
-			return key != null ? EncryptAsymmetricKeyParameter (key) : null;
+			return key != null ? (object) EncryptAsymmetricKeyParameter (key) : DBNull.Value;
 		}
 
 		static EncryptionAlgorithm[] DecodeEncryptionAlgorithms (DbDataReader reader, int column)
@@ -241,9 +241,6 @@ static EncryptionAlgorithm[] DecodeEncryptionAlgorithms (DbDataReader reader, in
 			var algorithms = new List<EncryptionAlgorithm> ();
 			var values = reader.GetString (column);
 
-			if (values.Equals ("None", StringComparison.OrdinalIgnoreCase))
-				return null;
-
 			foreach (var token in values.Split (new [] { ',' }, StringSplitOptions.RemoveEmptyEntries)) {
 				EncryptionAlgorithm algorithm;
 
@@ -263,10 +260,10 @@ static EncryptionAlgorithm[] DecodeEncryptionAlgorithms (DbDataReader reader, in
 			return algorithms.ToArray ();
 		}
 
-		static string EncodeEncryptionAlgorithms (EncryptionAlgorithm[] algorithms)
+		static object EncodeEncryptionAlgorithms (EncryptionAlgorithm[] algorithms)
 		{
 			if (algorithms == null || algorithms.Length == 0)
-				return "None";
+				return DBNull.Value;
 
 			var tokens = new string[algorithms.Length];
 			for (int i = 0; i < algorithms.Length; i++)