Merge branch 'beauby-resource-level-meta'

Followup concerns:
- https://github.com/rails-api/active_model_serializers/pull/1340/files#r47451387
- https://github.com/rails-api/active_model_serializers/pull/1340/files#r48116963
- https://github.com/rails-api/active_model_serializers/pull/1340#discussion_r47451387
- https://github.com/rails-api/active_model_serializers/pull/1340#issuecomment-164306882
- https://github.com/rails-api/active_model_serializers/pull/1340#issuecomment-166202978
- https://github.com/rails-api/active_model_serializers/pull/1340#issuecomment-173028896

test/adapter/json_api/resource_meta_test.rb

@@ -0,0 +1,63 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    module Adapter
+      class JsonApi
+        class ResourceMetaTest < Minitest::Test
+          class MetaHashPostSerializer < ActiveModel::Serializer
+            attributes :id
+            meta stuff: 'value'
+          end
+
+          class MetaBlockPostSerializer < ActiveModel::Serializer
+            attributes :id
+            meta do
+              { comments_count: object.comments.count }
+            end
+          end
+
+          def setup
+            @post = Post.new(id: 1337, comments: [], author: nil)
+          end
+
+          def test_meta_hash_object_resource
+            hash = ActiveModel::SerializableResource.new(
+              @post,
+              serializer: MetaHashPostSerializer,
+              adapter: :json_api
+            ).serializable_hash
+            expected = {
+              stuff: 'value'
+            }
+            assert_equal(expected, hash[:data][:meta])
+          end
+
+          def test_meta_block_object_resource
+            hash = ActiveModel::SerializableResource.new(
+              @post,
+              serializer: MetaBlockPostSerializer,
+              adapter: :json_api
+            ).serializable_hash
+            expected = {
+              comments_count: @post.comments.count
+            }
+            assert_equal(expected, hash[:data][:meta])
+          end
+
+          def test_meta_object_resource_in_array
+            hash = ActiveModel::SerializableResource.new(
+              [@post, @post],
+              each_serializer: MetaBlockPostSerializer,
+              adapter: :json_api
+            ).serializable_hash
+            expected = {
+              comments_count: @post.comments.count
+            }
+            assert_equal([expected, expected], hash[:data].map { |obj| obj[:meta] })
+          end
+        end
+      end
+    end
+  end
+end