Fixed S/MIME unit tests

It seems that PR #337 may be wrong as it causes a CastException
because identifier.Parameters is a DerInteger rather than a
DerSequence.

Restored my old logic for cases where identifier.Parameters
is not a DerSequence.

MimeKit/Cryptography/SecureMimeContext.cs

@@ -892,14 +892,25 @@ internal protected static bool TryGetEncryptionAlgorithm (AlgorithmIdentifier id
 			}
 
 			if (identifier.Algorithm.Id == CmsEnvelopedGenerator.RC2Cbc) {
-				var param = (DerSequence) identifier.Parameters;
-				var version = (DerInteger) param[0];
-				int bits = version.Value.IntValue;
-
-				switch (bits) {
-				case 58: algorithm = EncryptionAlgorithm.RC2128; return true;
-				case 120: algorithm = EncryptionAlgorithm.RC264; return true;
-				case 160: algorithm = EncryptionAlgorithm.RC240; return true;
+				if (identifier.Parameters is DerSequence) {
+					var param = (DerSequence) identifier.Parameters;
+					var version = (DerInteger) param[0];
+					int bits = version.Value.IntValue;
+
+					switch (bits) {
+					case 58: algorithm = EncryptionAlgorithm.RC2128; return true;
+					case 120: algorithm = EncryptionAlgorithm.RC264; return true;
+					case 160: algorithm = EncryptionAlgorithm.RC240; return true;
+					}
+				} else {
+					var param = (DerInteger) identifier.Parameters;
+					int bits = param.Value.IntValue;
+
+					switch (bits) {
+					case 128: algorithm = EncryptionAlgorithm.RC2128; return true;
+					case 64: algorithm = EncryptionAlgorithm.RC264; return true;
+					case 40: algorithm = EncryptionAlgorithm.RC240; return true;
+					}
 				}
 			}