Revert dropping MultiJson to fix Time serialization regression

[numbata]

Summary

This reverts the behavioral changes from #385 ("Drop multijson dependency") which introduced a silent breaking change in 1.0.3: all Time fields in API responses changed format for applications using ActiveSupport.

• Before (1.0.1): "2026-04-16T10:55:10.597Z" (ISO 8601)
• After (1.0.3): "2026-04-16 10:55:10 UTC"

The goal is to restore the exact serialization behavior from 1.0.1 for a patch release, so affected consumers can upgrade immediately without changes on their side.

What went wrong

PR #385 replaced MultiJson.dump with JSON.dump (via a Grape::Entity::Json shim) in Entity#to_json. These two methods behave differently when ActiveSupport is loaded:

• MultiJson.dump(hash) internally calls Hash#to_json, which ActiveSupport overrides to run as_json recursively — giving Time its ISO 8601 representation.
• JSON.dump(hash) goes directly to Ruby's C extension JSON generator, bypassing ActiveSupport's override — so Time falls back to Time#to_s.

Since activesupport is a runtime dependency of grape-entity, every user was affected.

Additionally, the Grape::Entity::Json shim from #385 checked defined?(::MultiJson) at require time to decide which backend to use. For applications with gem "multi_json", require: false, MultiJson was never loaded at that point — so the constant always resolved to ::JSON, making the MultiJson detection dead code in practice (as noted by @stevenou in #403).

What this PR does

• Restores require 'multi_json' and MultiJson.dump in Entity#to_json — the same code path that was in place at 1.0.1
• Re-adds multi_json as a gemspec dependency
• Removes the Grape::Entity::Json shim (lib/grape_entity/json.rb) and its spec, as they are no longer referenced
• Adds regression tests that assert Time values serialize via as_json in both flat and nested exposures

A separate PR (#404) explores dropping MultiJson entirely without the regression by using Hash#to_json directly — that is a candidate for a future minor release.

Fixes #403

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Pull request overview

Reverts the MultiJson removal from #385 to restore the pre-1.0.3 Time serialization behavior (ISO 8601 via ActiveSupport’s as_json) and prevent silent breaking changes in API responses.

Changes:

• Restored MultiJson.dump for Grape::Entity#to_json and reintroduced multi_json as a runtime dependency.
• Removed the Grape::Entity::Json shim and its spec since it is no longer used.
• Added regression specs to assert Time serialization via as_json for both flat and nested exposures.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
spec/grape_entity/json_spec.rb	Removes spec for the deleted Grape::Entity::Json shim.
spec/grape_entity/entity_spec.rb	Adds regression coverage for Time serialization (flat + nested).
lib/grape_entity/json.rb	Deletes the no-longer-used JSON backend shim.
lib/grape_entity/entity.rb	Restores MultiJson.dump in #to_json and requires multi_json.
grape-entity.gemspec	Re-adds multi_json as a dependency to support restored behavior.
CHANGELOG.md	Documents the regression fix/revert for the upcoming release.

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