Allow header field `Cache-Control` in preflight requests

Hopefully I can just code around this. However, I’m running into an issue with the Access-Control-Allow-Headers not including Cache-Control:

Failed to load https://mybinder.org/build/gh/nteract/vdom/master: Request header field Cache-Control is not allowed by Access-Control-Allow-Headers in preflight response

Full request structure (from Chrome dev tools):

:authority:mybinder.org
:method:OPTIONS
:path:/build/gh/nteract/vdom/master
:scheme:https
accept:*/*
accept-encoding:gzip, deflate, br
accept-language:en-US,en;q=0.9
access-control-request-headers:cache-control
access-control-request-method:GET
cache-control:no-cache
origin:http://127.0.0.1:3000
pragma:no-cache
referer:http://127.0.0.1:3000/
user-agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.94 Safari/537.36

[Miniland1333]

I’ve removed the requestKernel: true, which means that binder instances are now manually requested only. I might integrate thebelab into my analytics to gauge how popular it is.

Thebelab itself does not use serviceworkers but the site as a whole does. I’m glad we could deal with the issue binderhub-side since the serviceworkers are bundled as part of the PaaS.

[Miniland1333]

I’m using @minrk 's Thebelab project to embed into an HTML webpage. I knew of Thebe but didn’t know Thebelab had resurrected the project. I wish Thebelab had higher visibility since it is perfect for external websites to communicate with Binderhub!

An example of Thebelab that I’m working on is Chuck Serverance’s e-textbook. (You many need to refresh the page once due to the above issue)