parsing derivative manifest gets original manifest @id

[saracarl]

When I parse a derivative manifest, the service['@id'] returns the original manifest @id, not the derivative manifest @id.

I'm working with a derivative manifest -- in this example, a book with some of the front matter removed.

The original manifest is here:
http://gallica.bnf.fr/iiif/ark:/12148/btv1b52505635t/manifest.json

I edited it with the Bodlein's IIIF Manifest editor and removed the first couple of pages. I exported the resulting manifest and stuck it on Gist here:
https://gist.githubusercontent.com/saracarl/bc089a79cf871af077ecc07abaced90e/raw/271c28dc9ff07573c7dd42af78bb56f5da91e033/gallica_der_manifest.json

and edited the @id to refer to itself:
"@id": "https://gist.githubusercontent.com/saracarl/bc089a79cf871af077ecc07abaced90e/raw/b369d953395343e9dfa17a578762a1bc5392d282/gallica_der_manifest.json",

I then try to work with the derivative manifest. I open our derivative @id and parse it using IIIF::Service.parse. When I ask for the @id of the parsed manifest, it returns the original manifest @id (http://gallica.bnf.fr/iiif/ark:/12148/btv1b52505635t/manifest.json).

connection = open(https://gist.githubusercontent.com/saracarl/bc089a79cf871af077ecc07abaced90e/raw/b369d953395343e9dfa17a578762a1bc5392d282/gallica_der_manifest.json)
manifest_json = connection.read
service = IIIF::Service.parse(manifest_json)
service['@id']

returns http://gallica.bnf.fr/iiif/ark:/12148/btv1b52505635t/manifest.json

When I double check the gist file, I confirm that http://gallica.bnf.fr/iiif/ark:/12148/btv1b52505635t/manifest.json isn't anywhere in the file.

It's an easy fix from my end -- I'm just going to set the @id of my working manifest to the passed @id, but either there is a bug in the code, or my understanding.

I'll point out that Mirador works fine here with the derivative manifest, but perhaps they aren't parsing the manifest the same way?

I've reproduced this at .10 and .20 versions of the gem.