Allow redirects to redirect, keeping the same endpoint

[kmccarp]

Redirect 3xx status codes with a Location header to http://httpstat.us/200 for testing that your application handles redirects (and keeps headers) correctly.

[aaronpowell]

Seems like a reasonable idea, being able to dictate the next step in the redirect, so if the PR updates to reflect that I think it makes the most sense.

Right now, it's a bit limited value, I guess that it returns JSON. I wonder if we should make the home page available via JSON as well...

[daraul]

I found myself having to debug a max-redirects exception today, and immediately thought it'd make sense to use https://httpstat.us/302 with some trickery to make it redirect to itself n times.

[aaronpowell]

Closing this as it's better solved by providing a custom Location in the request custom headers.