Gateway: Possible Invalid 'X-Ipfs-Roots' Header

[hacdias]

Extracted from #176 (comment) to avoid blocking #176.

⚠️ since pathMetadata is based on maybeResolvedImPath, if the path is indeed resolved to the final /ipfs/cid, we will produce an invalid X-Ipfs-Roots header that does not include logical parents from immutableContentPath.

Example:

• immutableContentPath – /ipfs/cid/foo/bar
• maybeResolvedImPath – /ipfs/bar-cid
• X-Ipfs-Roots
  • expected: cid, foo-cid, bar-cid
  • produced: bar-cid

Idea

Maybe some struct that gets passed around, like:

type requestData struct {
	// Defined for all requests.
	begin             time.Time
	logger            *zap.SugaredLogger
	contentPath       ipath.Path
	contentType       string
	contentTypeParams map[string]string

	// Defined for non IPNS Record requests.
	immutablePath ImmutablePath

	// Defined if resolution has already happened.
	pathMetadata *ContentPathMetadata
	resolvedPath *ImmutablePath
}

func (rq *requestData) maybeResolvedPath() ImmutablePath {
	if rq.resolvedPath != nil {
		return *rq.resolvedPath
	}
	return rq.immutablePath
}

### Tasks
- [ ] confirm header works as expected or there is a bug (if so, fix it)
- [ ] add regression test to https://github.com/ipfs/gateway-conformance for the Example above