Relax the mimetype check of the .well-known/web-identity file

[samuelgoto]

The algorithm for fetching and parsing the .well-known file enforces that the mime type must be text/json here.

Unfortunately, there are many occasions where developers don't have the ability to control the mime type in their hosting servers. For example, everybody using github pages can't serve an text/json file unless it matches the predefined extension, .js.

https://docs.github.com/en/pages/getting-started-with-github-pages/about-github-pages#mime-types-on-github-pages

Since .well-known/web-identity doesn't have a file extension, the server serves application/octet-stream instead by default.

For example:

curl http://sgo.to/.well-known/web-identity --verbose

Ideally, unless something else breaks, we should relax the requirement of mime types, and be more accepting of different HTTP responses, as long as we can get the providers_urls list.

I ran into this while trying to set up #2 .

I think this could also be solved with #9.