Merge PR icsharpcode#325: Ability to mimic a zip file created by a Linux file-system

* respect the VersionMadeBy property of ZipEntry
* correctly set HostSystem without clearing version

(cherry picked from commit 43fd81e)

Author: bjorg

ICSharpCode.SharpZipLib/Zip/ZipEntry.cs

@@ -471,7 +471,7 @@ public int HostSystem {
 			}
 
 			set {
-				versionMadeBy &= 0xff;
+				versionMadeBy &= 0x00ff;
 				versionMadeBy |= (ushort)((value & 0xff) << 8);
 			}
 		}

ICSharpCode.SharpZipLib/Zip/ZipFile.cs

@@ -1876,7 +1876,7 @@ int WriteCentralDirectoryHeader(ZipEntry entry)
 			WriteLEInt(ZipConstants.CentralHeaderSignature);
 
 			// Version made by
-			WriteLEShort(ZipConstants.VersionMadeBy);
+			WriteLEShort((entry.HostSystem << 8) | entry.VersionMadeBy);
 
 			// Version required to extract
 			WriteLEShort(entry.Version);

ICSharpCode.SharpZipLib/Zip/ZipOutputStream.cs

@@ -647,7 +647,7 @@ public override void Finish()
 
 			foreach (ZipEntry entry in entries) {
 				WriteLeInt(ZipConstants.CentralHeaderSignature);
-				WriteLeShort(ZipConstants.VersionMadeBy);
+				WriteLeShort((entry.HostSystem << 8) | entry.VersionMadeBy);
 				WriteLeShort(entry.Version);
 				WriteLeShort(entry.Flags);
 				WriteLeShort((short)entry.CompressionMethodForHeader);