Variable IMF_ACCEPT_HEADER`Const`

IMF_ACCEPT_HEADER: "application/json, application/vnd.sdmx.data+json, */*;q=0.8" = 'application/json, application/vnd.sdmx.data+json, */*;q=0.8'

IMF SDMX accepts JSON data; keep a fallback for proxy/content negotiation.