Don't remove `ProjectSnippet#expires_at` from API

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3103

This partially reverts commit 836d5930.

doc/api/notes.md

@@ -145,6 +145,7 @@ Parameters:
     "state": "active",
     "created_at": "2013-09-30T13:46:01Z"
   },
+  "expires_at": null,
   "updated_at": "2013-10-02T07:34:20Z",
   "created_at": "2013-10-02T07:34:20Z"
 }

doc/api/project_snippets.md

@@ -51,6 +51,7 @@ Parameters:
     "state": "active",
     "created_at": "2012-05-23T08:00:58Z"
   },
+  "expires_at": null,
   "updated_at": "2012-06-28T10:52:04Z",
   "created_at": "2012-06-28T10:52:04Z"
 }

doc/web_hooks/web_hooks.md

@@ -582,6 +582,7 @@ X-Gitlab-Event: Note Hook
     "created_at": "2015-04-09 02:40:38 UTC",
     "updated_at": "2015-04-09 02:40:38 UTC",
     "file_name": "test.rb",
+    "expires_at": null,
     "type": "ProjectSnippet",
     "visibility_level": 0
   }

lib/api/entities.rb

@@ -144,6 +144,9 @@ module API
       expose :id, :title, :file_name
       expose :author, using: Entities::UserBasic
       expose :updated_at, :created_at
+
+      # TODO (rspeicher): Deprecated; remove in 9.0
+      expose(:expires_at) { |snippet| nil }
     end
     class ProjectEntity < Grape::Entity

spec/requests/api/project_snippets_spec.rb

+require 'rails_helper'
+
+describe API::API, api: true do
+  include ApiHelpers
+
+  describe 'GET /projects/:project_id/snippets/:id' do
+    # TODO (rspeicher): Deprecated; remove in 9.0
+    it 'always exposes expires_at as nil' do
+      admin = create(:admin)
+      snippet = create(:project_snippet, author: admin)
+
+      get api("/projects/#{snippet.project.id}/snippets/#{snippet.id}", admin)
+
+      expect(json_response).to have_key('expires_at')
+      expect(json_response['expires_at']).to be_nil
+    end
+  end
+end