Use append-method instead of "new Uri(BaseUri, url);"

[StefH]

This fixes the issue when you provide a basePath which is not in the same folder as the htm file, like this:

string packaged = packager.PackageHtml(@"c:\temp\web\github.htm", @"c:\temp\web\GitHub_files");

[StefH]

@RickStrahl Please take a look at this PR.

More details also here: https://stackoverflow.com/questions/372865/path-combine-for-urls