make_callback does not support preflight requests for CORS

[emillon]

Hi,

When the client is on a different origin than the server, the client sends a preflight OPTIONS request to OGS, which returns a 404:

ocaml-graphql-server/graphql-cohttp/src/graphql_cohttp.ml

Line 188 in 940e86f

| _ -> respond_string ~status:`Not_found ~body:"" ()

That prevents directly using make_callback in this scenario.

What do you think about adding a ?allowed_origins optional parameter that adds the right handler to OPTIONS /graphql? I'm happy to provide a patch.

Thanks!

[andreas]

Yes, that would be great 😄