Merge PR icsharpcode#312: Set the value of CompressionMethod.BZip2 to 12 as per spec

See: https://www.pkware.com/documents/casestudies/APPNOTE.TXT paragraph 4.4.5

Author: Numpsy

src/ICSharpCode.SharpZipLib/Zip/ZipConstants.cs

@@ -50,7 +50,7 @@ public enum CompressionMethod
 		/// <summary>
 		/// BZip2 compression. Not supported by #Zip.
 		/// </summary>
-		BZip2 = 11,
+		BZip2 = 12,
 
 		/// <summary>
 		/// WinZip special for AES encryption, Now supported by #Zip.