Clear POP3 capabilities in QueryCapabilities instead of the CAPA call…

…back handler

This way, if the server replies with -ERR in a subsequent CAPA query,
all capabilities are cleared. If the clearing happens in the callback
handler, then they are only updated if the server replies with a +OK
response.

Fixes issue #174

MailKit/Net/Pop3/Pop3Engine.cs

@@ -445,13 +445,6 @@ public Pop3Command QueueCommand (CancellationToken cancellationToken, Pop3Comman
 
 		static void CapaHandler (Pop3Engine engine, Pop3Command pc, string text)
 		{
-			// clear all CAPA response capabilities (except the APOP capability)
-			engine.Capabilities &= Pop3Capabilities.Apop;
-			engine.AuthenticationMechanisms.Clear ();
-			engine.Implementation = null;
-			engine.ExpirePolicy = 0;
-			engine.LoginDelay = 0;
-
 			if (pc.Status != Pop3CommandStatus.Ok)
 				return;
 
@@ -542,6 +535,13 @@ public Pop3CommandStatus QueryCapabilities (CancellationToken cancellationToken)
 			if (stream == null)
 				throw new InvalidOperationException ();
 
+			// clear all CAPA response capabilities (except the APOP capability)
+			Capabilities &= Pop3Capabilities.Apop;
+			AuthenticationMechanisms.Clear ();
+			Implementation = null;
+			ExpirePolicy = 0;
+			LoginDelay = 0;
+
 			var pc = QueueCommand (cancellationToken, CapaHandler, "CAPA");
 
 			while (Iterate () < pc.Id) {