SPIKE: Get on upstream version of httpbin

[EdVinyard]

Stoplight's httpbin fork is broken with latest version of Flask

Either...

1. Stop using a fork of httpbin for testing.
2. Update forked httpbin code to work with latest version of Flask (e.g., no more JSONEncoder).

https://github.com/stoplightio/httpbin/pull/2 was the original workaround.

What questions to answer:

1. what changed in our fork that the upstream doesn't have?
2. are any of those critical still?
3. how much work would it take to build in any features we need to retain in the upstream repo?