The RFC <https://tools.ietf.org/html/rfc7231#page-39> says “A request without any Accept header field implies that the user agent will accept any media type in response”, which sounds the same as “Accept: */*”. I don’t understand why adding it should make a real difference.

If you really desire only application/json, you should probably include “Accept: application/json” in the request. Otherwise, it would probably be more robust to make your program accept both types. I have come across the same deal with application/atom+xml vs text/xml vs application/xml.