Null ref in RemotePackageWalker

[davidfowl]

http://projectk-tc/viewLog.html?buildId=11729&buildTypeId=WebListener&tab=buildLog#_focus=2440

/cc @loudej

Looks like restore failed here

[davidfowl]

I worked around it to unblock coherence but it might be a case sensitivity thing. See the diff that fixed it here:

aspnet/HttpSysServer@21616bc

[jeremymeng]

When downloading zip files it seems working regardless of the casing. However when getting the .nuspec file from the downloaded zip file, it is case-sensitive. We should give users proper error message instead of a null ref exception.

        public async Task<Stream> OpenNuspecStreamAsync(PackageInfo package)
        {
            using (var nupkgStream = await OpenNupkgStreamAsync(package))
            {
                using (var archive = new ZipArchive(nupkgStream, ZipArchiveMode.Read, leaveOpen: true))
                {
                    var entry = archive.GetEntry(package.Id + ".nuspec");
                    using (var entryStream = entry.Open())
                    {