Fixed logic for formatting IMAP FETCH HEADER.FIELDS.NOT corner case

If IFetchRequest.Headers excludes References but IFetchRequest.Items includes References,
the formatted HEADER.FIELDS.NOT result contain REFERENCES, but it really shouldn't.

This commit fixes it so that IFetchRequest.Items References overrides this
such that the References exclude will be removed.

MailKit/Net/Imap/ImapFolderFetch.cs

@@ -722,6 +722,11 @@ Task UntaggedFetchSummaryItemsHandler (ImapEngine engine, ImapCommand ic, int in
 			return Task.CompletedTask;
 		}
 
+		static bool IsEmptyExclude (HeaderSet headers, MessageSummaryItems items)
+		{
+			return headers.Exclude && (headers.Count == 0 || headers.Count == 1 && (items & MessageSummaryItems.References) != 0);
+		}
+
 		internal static string FormatSummaryItems (ImapEngine engine, IFetchRequest request, out bool previewText, bool isNotify = false)
 		{
 			var items = request.Items;
@@ -816,12 +821,15 @@ internal static string FormatSummaryItems (ImapEngine engine, IFetchRequest requ
 			}
 
 			if (request.Headers != null) {
-				if (request.Headers.Count == 0 && request.Headers.Exclude) {
+				if (IsEmptyExclude (request.Headers, request.Items)) {
 					tokens.Add ("BODY.PEEK[HEADER]");
 				} else if (request.Headers.Exclude) {
 					var headerFields = new StringBuilder ("BODY.PEEK[HEADER.FIELDS.NOT (");
 
 					foreach (var header in request.Headers) {
+						if ((request.Items & MessageSummaryItems.References) != 0 && header.Equals ("REFERENCES", StringComparison.Ordinal))
+							continue;
+
 						headerFields.Append (header);
 						headerFields.Append (' ');
 					}

UnitTests/Net/Imap/ImapFolderFetchTests.cs

@@ -2038,5 +2038,48 @@ public void TestFormatFetchSummaryItemsAllHeaders ()
 				Assert.That (command, Is.EqualTo ("BODY.PEEK[HEADER]"));
 			}
 		}
+
+		[Test]
+		public void TestFormatFetchSummaryItemsHeaderFieldsAndReferences ()
+		{
+			using (var engine = new ImapEngine (null)) {
+				var request = new FetchRequest (MessageSummaryItems.References) {
+					Headers = new HeaderSet (new[] { HeaderId.InReplyTo })
+				};
+
+				var command = ImapFolder.FormatSummaryItems (engine, request, out var previewText);
+				Assert.That (command, Is.EqualTo ("BODY.PEEK[HEADER.FIELDS (IN-REPLY-TO REFERENCES)]"));
+			}
+		}
+
+		[Test]
+		public void TestFormatFetchSummaryItemsExcludeHeaderFieldsReferencesAndReferences ()
+		{
+			using (var engine = new ImapEngine (null)) {
+				var request = new FetchRequest (MessageSummaryItems.References) {
+					Headers = new HeaderSet (new[] { HeaderId.References }) {
+						Exclude = true
+					}
+				};
+
+				var command = ImapFolder.FormatSummaryItems (engine, request, out var previewText);
+				Assert.That (command, Is.EqualTo ("BODY.PEEK[HEADER]"));
+			}
+		}
+
+		[Test]
+		public void TestFormatFetchSummaryItemsExcludeHeaderFieldsInReplyToReferencesAndReferences ()
+		{
+			using (var engine = new ImapEngine (null)) {
+				var request = new FetchRequest (MessageSummaryItems.References) {
+					Headers = new HeaderSet (new[] { HeaderId.InReplyTo, HeaderId.References }) {
+						Exclude = true
+					}
+				};
+
+				var command = ImapFolder.FormatSummaryItems (engine, request, out var previewText);
+				Assert.That (command, Is.EqualTo ("BODY.PEEK[HEADER.FIELDS.NOT (IN-REPLY-TO)]"));
+			}
+		}
 	}
 }