PR #522: Use PackageIcon instead of PackageIconUrl in csproj

* Update ICSharpCode.SharpZipLib.csproj
* Add nuget icon
* Fix assets path

src/ICSharpCode.SharpZipLib/ICSharpCode.SharpZipLib.csproj

@@ -20,7 +20,7 @@
     <Description>SharpZipLib (#ziplib, formerly NZipLib) is a compression library for Zip, GZip, BZip2, and Tar written entirely in C# for .NET. It is implemented as an assembly (installable in the GAC), and thus can easily be incorporated into other projects (in any .NET language)</Description>
 		<PackageLicenseExpression>MIT</PackageLicenseExpression>
 		<PackageProjectUrl>http://icsharpcode.github.io/SharpZipLib/</PackageProjectUrl>
-		<PackageIconUrl>http://icsharpcode.github.io/SharpZipLib/assets/sharpziplib-nuget-256x256.png</PackageIconUrl>
+		<PackageIcon>images/sharpziplib-nuget-256x256.png</PackageIcon>
     <RepositoryUrl>https://github.com/icsharpcode/SharpZipLib</RepositoryUrl>
     <Copyright>Copyright © 2000-2020 SharpZipLib Contributors</Copyright>
     <PackageTags>Compression Library Zip GZip BZip2 LZW Tar</PackageTags>
@@ -33,5 +33,12 @@ Please see https://github.com/icsharpcode/SharpZipLib/wiki/Release-1.3 for more
 	<ItemGroup>
 		<PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
 	</ItemGroup>
+
+	 <ItemGroup>
+	   <None Include="../../assets/sharpziplib-nuget-256x256.png">
+      <Pack>True</Pack>
+      <PackagePath>images</PackagePath>
+    </None>
+	 </ItemGroup>
 
 </Project>