Support for headers: Accept and If-Modified-Since

[juztas]

While reviewing OpenAPI Spec (https://github.com/doe-iri/iri-facility-api-docs/blob/main/specification/openapi/openapi_iri_facility_api_v1.yaml#L154-L169) I noticed that python implementation does not implement (nor expose in OpenAPI.json) any of the headers (Accept and If-Modified-Since).

• Accept - application/json. Easy to add;
• If-Modified-Since (header) and modified_since (query parameter). This allows client to specify one or both. What is the correct precedence rule if both are specified? I think we need to define it explicitly in the API Spec the expected behavior and implement it inside the API. Something like:

If both the If-Modified-Since header and the modified_since query parameter are provided, the modified_since query parameter takes priority.

or

Use the most recent timestamp between the If-Modified-Since header and the modified_since query parameter — whichever is later takes precedence.

@jmacauley