fix(tar): read full extended headers (#675)

* fix(tar): read full extended headers

* Update src/ICSharpCode.SharpZipLib/Tar/TarExtendedHeaderReader.cs

src/ICSharpCode.SharpZipLib/Tar/TarExtendedHeaderReader.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Text;
 
 namespace ICSharpCode.SharpZipLib.Tar
@@ -26,7 +27,10 @@ public class TarExtendedHeaderReader
 
 		private int state = LENGTH;
 
-		private static readonly byte[] StateNext = new[] { (byte)' ', (byte)'=', (byte)'\n' };
+		private int currHeaderLength;
+		private int currHeaderRead;
+
+		private static readonly byte[] StateNext = { (byte)' ', (byte)'=', (byte)'\n' };
 
 		/// <summary>
 		/// Creates a new <see cref="TarExtendedHeaderReader"/>.
@@ -46,23 +50,46 @@ public void Read(byte[] buffer, int length)
 			for (int i = 0; i < length; i++)
 			{
 				byte next = buffer[i];
+
+				var foundStateEnd = state == VALUE 
+					? currHeaderRead == currHeaderLength -1
+					: next == StateNext[state];
 
-				if (next == StateNext[state])
+				if (foundStateEnd)
 				{
 					Flush();
 					headerParts[state] = sb.ToString();
 					sb.Clear();
-
+					
 					if (++state == END)
 					{
-						headers.Add(headerParts[KEY], headerParts[VALUE]);
+						if (!headers.ContainsKey(headerParts[KEY]))
+						{
+							headers.Add(headerParts[KEY], headerParts[VALUE]);
+						}
+
 						headerParts = new string[3];
+						currHeaderLength = 0;
+						currHeaderRead = 0;
 						state = LENGTH;
 					}
+					else
+					{
+						currHeaderRead++;
+					}
+
+
+					if (state != VALUE) continue;
+
+					if (int.TryParse(headerParts[LENGTH], out var vl))
+					{
+						currHeaderLength = vl;
+					}
 				}
 				else
 				{
 					byteBuffer[bbIndex++] = next;
+					currHeaderRead++;
 					if (bbIndex == 4)
 						Flush();
 				}