Fixed message reserialization after prepending headers

Fixes issue #524

MimeKit/MimeMessage.cs

@@ -2143,6 +2143,17 @@ IEnumerable<Header> MergeHeaders ()
 		{
 			int mesgIndex = 0, bodyIndex = 0;
 
+			// write all of the prepended message headers first
+			while (mesgIndex < Headers.Count) {
+				var mesgHeader = Headers[mesgIndex];
+				if (mesgHeader.Offset.HasValue)
+					break;
+
+				yield return mesgHeader;
+				mesgIndex++;
+			}
+
+			// now merge the message and body headers as they appeared in the raw message
 			while (mesgIndex < Headers.Count && bodyIndex < Body.Headers.Count) {
 				var bodyHeader = Body.Headers[bodyIndex];
 				if (!bodyHeader.Offset.HasValue)

UnitTests/MimeMessageTests.cs

@@ -104,6 +104,40 @@ public void TestArgumentExceptions ()
 			Assert.Throws<ArgumentNullException> (() => MimeMessage.CreateFromMailMessage (null));
 		}
 
+		[Test]
+		public void TestPrependHeader ()
+		{
+			string rawMessageText = @"Date: Fri, 22 Jan 2016 8:44:05 -0500 (EST)
+From: MimeKit Unit Tests <[email protected]>
+To: MimeKit Unit Tests <[email protected]>
+Subject: This is a test off prepending headers.
+Message-Id: <[email protected]>
+MIME-Version: 1.0
+Content-Type: text/plain
+
+This is the message body.
+".Replace ("\r\n", "\n");
+			string expected = "X-Prepended: This is the prepended header\n" + rawMessageText;
+
+			using (var source = new MemoryStream (Encoding.UTF8.GetBytes (rawMessageText))) {
+				var parser = new MimeParser (source, MimeFormat.Default);
+				var message = parser.ParseMessage ();
+
+				message.Headers.Insert (0, new Header ("X-Prepended", "This is the prepended header"));
+
+				using (var serialized = new MemoryStream ()) {
+					var options = FormatOptions.Default.Clone ();
+					options.NewLineFormat = NewLineFormat.Unix;
+
+					message.WriteTo (options, serialized);
+
+					var result = Encoding.UTF8.GetString (serialized.ToArray ());
+
+					Assert.AreEqual (expected, result, "Reserialized message is not identical to the original.");
+				}
+			}
+		}
+
 		[Test]
 		public async Task TestReserialization ()
 		{