bundlerApp: Set up passthru properly

[lilyball]

Motivation for this change

The manual documents both bundlerEnv and bundlerApp as providing env and wrappedRuby attributes on the resulting derivations. However only bundlerEnv actually had these attributes. Update bundlerApp to provide the same passthru that bundlerEnv does.

Since bundlerApp already let the user provide their own passthru, I've chosen to allow that to take precedence over the default passthru, meaning the user may overwrite env or wrappedRuby with their own attributes if they choose to do so.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

I have not actually tested the impact of this change on the nixpkgs repository. It shouldn't result in rebuilding any bundlerApp packages as it only affects passthru. I did test the one package I actually care about (the package whose environment I want access to in a nix-shell). Given that any manually-provided passthru argument will overwrite what I'm adding, this change should be purely additive.

[nixos-discourse]

This pull request has been mentioned on Nix community. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review-may-2019/3032/19