Double encoded post requests after recent update (4.89.3 → 4.91.1)

[finestructure]

Our social media posts made via

        struct Query: Encodable {
            var status: String
        }

        let res = try await client.post(URI(string: apiURL), headers: headers) { req in
            try req.query.encode(Query(status: message))
        }

started showing up as

after updating from Vapor 4.89.3 → 4.91.1 today.

@gwynne seems to know what the issue might be, adding the issue for tracking purposes.

[gwynne]

Duping to #3138, the central tracking for the underlying issue.