Don't use FormatOptons to hold temporary state when writing out messages

Added an internal HeaderList.Suppress property to hold the state instead.

This is a far better fix for issue #138

MimeKit/FormatOptions.cs

@@ -131,10 +131,6 @@ internal byte[] NewLineBytes {
 			get { return NewLineFormats[(int) NewLineFormat]; }
 		}
 
-		internal bool WriteHeaders {
-			get; set;
-		}
-
 		/// <summary>
 		/// Gets the message headers that should be hidden.
 		/// </summary>
@@ -208,7 +204,6 @@ public FormatOptions ()
 			HiddenHeaders = new HashSet<HeaderId> ();
 			//maxLineLength = DefaultMaxLineLength;
 			AllowMixedHeaderCharsets = true;
-			WriteHeaders = true;
 
 			if (Environment.NewLine.Length == 1)
 				newLineFormat = NewLineFormat.Unix;
@@ -231,7 +226,6 @@ public FormatOptions Clone ()
 			options.HiddenHeaders = new HashSet<HeaderId> (HiddenHeaders);
 			options.AllowMixedHeaderCharsets = AllowMixedHeaderCharsets;
 			options.international = international;
-			options.WriteHeaders = true;
 			return options;
 		}
 

MimeKit/HeaderList.cs

@@ -49,8 +49,10 @@ public sealed class HeaderList : IList<Header>
 	{
 		static readonly StringComparer icase = StringComparer.OrdinalIgnoreCase;
 
-		// this table references the first header of each field
 		internal readonly ParserOptions Options;
+		internal bool Suppress;
+
+		// this table references the first header of each field
 		readonly Dictionary<string, Header> table;
 		readonly List<Header> headers;
 
@@ -655,6 +657,9 @@ public string this [string field] {
 			if (stream == null)
 				throw new ArgumentNullException ("stream");
 
+			if (Suppress)
+				return;
+
 			using (var filtered = new FilteredStream (stream)) {
 				filtered.Add (options.CreateNewLineFilter ());
 

MimeKit/MimeEntity.cs

@@ -379,10 +379,7 @@ public virtual void Accept (MimeVisitor visitor)
 			if (stream == null)
 				throw new ArgumentNullException ("stream");
 
-			if (options.WriteHeaders)
-				Headers.WriteTo (options, stream, cancellationToken);
-			else
-				options.WriteHeaders = true;
+			Headers.WriteTo (options, stream, cancellationToken);
 
 			var cancellable = stream as ICancellableStream;
 

MimeKit/MimeMessage.cs

@@ -954,8 +954,12 @@ public virtual void Accept (MimeVisitor visitor)
 					filtered.Flush (cancellationToken);
 				}
 
-				options.WriteHeaders = false;
-				Body.WriteTo (options, stream, cancellationToken);
+				try {
+					Body.Headers.Suppress = true;
+					Body.WriteTo (options, stream, cancellationToken);
+				} finally {
+					Body.Headers.Suppress = false;
+				}
 			}
 		}