Expose a way to passthrough after the request is found to be unhandled

[happycollision]

This PR aims to allow a request to be passed-through in the unhandledRequests hook.

Totally fine to say that you don't want this feature. I am adding it for me, but since it was simple, I figured add PR you here.

This allows you to actually pass certain unhandled requests through at runtime if you so choose. Sam Selikoff offered this as an example of a feature this unlocks (better than my original example). Currently, you can't really simply declare multiple multiple domains as a passthrough. You'd need something like:

this.get('api1.something.com', this.passthrough)
this.get('api2.something.com', this.passthrough)
// etc

This feature addition allows you to handle all of them with any runtime check you'd like to implement:

server.unhandledRequest = function(verb, path, request) {
  if (path.startsWith('aws')) {
    request.passthrough() // <-- This is what is now possible
  }
}

• Add a .passthrough() method to the FakeRequest
• Add tests.
• Add documentation.

[happycollision]

I created this PR a long time ago with the full intent to finish off that checklist above. However, I understand that new features like this may not be desirable in your project, and now that I am back to working on a project that depends on my fork, I'd love to finish this off if it is something that belongs in Pretender. I'd be happy to finish this PR (and also add proper TS typings for the new feature) if someone who maintains this project would let me know if the functionality is desired. I found it immensely helpful to be able to call .passthrough() on a request as illustrated in my example code above.

Any thoughts, @bantic, @mike-north, @trek, or @xg-wang?

[happycollision]

@xg-wang I went ahead and did this. Tests pass. I hope this feature is something you agree with. I have found it immensely useful.

[samselikoff]

Being able to decide whether to passthrough a request at runtime is super useful & actually something we added to Mirage. I'm in favor!

[xg-wang]

Yeah I think this feature is great, and your use case makes a lot of sense. Sorry for the delay!

[happycollision]

I am happy to make the requested changes. I don’t blame you for not commenting sooner. My original example left a little something to be desired (like how is it useful?!).

Thanks for having a look. I’ll get around to those changes either tonight or within a few days.

[happycollision]

I should note that in addition to the adding a test, I altered the readme to more accurately reflect what happens when the user calls the new method.