fetch missing *gemspec.rz files

To get a local mirror working, I had problems with missing gemspec.rz files under the quick/Marshal directory. The original assumed that a gemspec file was downloaded along with the original gem, but I'd managed to lose some earlier gemspec files and didn't want to redownload the entire (currently 257k gems) collection.

I wanted to submit my changes to the rubygems/rubygems-mirror, but they don't seem to have accepted your pull request, and I based my work on the excellent workaround you'd done to avoid attempting to rebuild the index.  Thanks for your work.