JSON-encode GlobalIDs as strings

[georgeclaghorn]

Before:

> Plan.last.to_gid.as_json
{"uri"=>"gid://sandbox/Plan/186"}

> Plan.last.to_gid.to_json
"{\"uri\":\"gid://sandbox/Plan/186\"}"

After:

> Plan.last.to_gid.as_json
"gid://sandbox/Plan/186"

> Plan.last.to_gid.to_json
"\"gid://sandbox/Plan/186\""

This is particularly useful for JSON-serialized attributes in Rails, which use the result of #as_json on assigned values to generate the JSON they store.

class SubscriptionChange < ApplicationRecord
  serialize :old_product_gids, JSON
end

Before:

> SubscriptionChange.create!(old_product_gids: [ Plan.last.to_gid ]).old_product_gids
# => [{ "uri" => "gid://sandbox/Plan/186" }]

After:

> SubscriptionChange.create!(old_product_gids: [ Plan.last.to_gid ]).old_product_gids
# => [ "gid://sandbox/Plan/186" ]

[dhh]

Looks good!