Make sure to set proper NTLM flags when setting Domain and Host for Type3Messages

Also don't override the Domain if the user-supplied domain iss empty.

*May* fix issue #532

Author: jstedfast

MailKit/Security/Ntlm/Type3Message.cs

@@ -62,9 +62,9 @@ public Type3Message (Type2Message type2, string userName, string hostName) : bas
 			Level = NtlmSettings.DefaultAuthLevel;
 
 			challenge = (byte[]) type2.Nonce.Clone ();
-			domain = type2.TargetName;
+			Domain = type2.TargetName;
 			Username = userName;
-			host = hostName;
+			Host = hostName;
 
 			Flags = (NtlmFlags) 0x8200;
 			if ((type2.Flags & NtlmFlags.NegotiateUnicode) != 0)

MailKit/Security/SaslMechanismNtlm.cs

@@ -149,7 +149,9 @@ static MessageBase GetChallengeResponse (string userName, string password, strin
 			var type2 = new Type2Message (token, startIndex, length);
 			var type3 = new Type3Message (type2, userName, string.Empty);
 			type3.Password = password;
-			type3.Domain = domain;
+
+			if (!string.IsNullOrEmpty (domain))
+				type3.Domain = domain;
 
 			return type3;
 		}