fix: implicit accept header can be overridden by format query

SgtPooki commented 7 months ago

Title

Description

Fixes https://github.com/ipfs/helia-verified-fetch/issues/35 & also allows mapping of 'raw' to 'dag-json'. I imagine users will get errors if they try to accept dag-json for non-json content, we can address that later?

Notes & open questions

Basically, we were previously doing the following:

check accept header
check format query param
if accept header exists, use it, otherwise, use the mapped query param format

However, that was causing the bug in https://github.com/ipfs/helia-verified-fetch/issues/35 because the format query param was being ignored if the accept header was present (which it usually is for most clients), even if it wasn't a valid IPFS gateway Accept header

Now, we do this:

check accept header
check format query param
if accept header is not a valid IPFS gateway accept header, and we have a mapping for the format query param, use the mapped query param format value.
otherwise use the accept header

more notes are in https://github.com/ipfs/helia-verified-fetch/issues/35

Change checklist

[ ] I have performed a self-review of my own code
[ ] I have made corresponding changes to the documentation if necessary (this includes comments as well)
[x] I have added tests that prove my fix is effective or that my feature works