Grape formatter feature requested in rails-api#1268

 - adds handling for when the returned resource is not serializable via ams
 - fix for when resource is an Array
 - fixing last checkin
 - Adds intellij .idea file to .gitignore
 - Moves grape include to grape namespace. Changes Enumerable to Array because a plain hash is enumerable.
 - rubocop corrections
 - Rebased and squashed commits, resubmitted to CI

.gitignore

@@ -21,3 +21,4 @@ tmp
 *.swp
 .ruby-version
 .ruby-gemset
+.idea

lib/grape/active_model_serializers.rb

@@ -0,0 +1,6 @@
+# To add grape support, require 'grape/active_model_serializers' in the base of your grape endpoints
+# Then add 'formatter :json, Grape::Formatters::ActiveModelSerializers' to the endpoints
+# Then add 'helpers Grape::Helpers::ActiveModelSerializers' to the endpoints
+require 'active_model_serializers'
+require 'grape/formatters/active_model_serializers'
+require 'grape/helpers/active_model_serializers'

lib/grape/formatters/active_model_serializers.rb

@@ -0,0 +1,18 @@
+# A grape response formatter that can be used as 'formatter :json, Grape::Formatters::ActiveModelSerializers'
+#
+# Serializer options can be passed as a hash from your grape endpoint using env[:active_model_serializer_options],
+# or better yet user the render helper in Grape::Helpers::ActiveModelSerializers
+module Grape
+  module Formatters
+    module ActiveModelSerializers
+      class << self
+        def call(resource, env)
+          return resource.to_json unless resource.is_a?(Array) || resource.respond_to?(:model_name)
+          serializer_options = {}
+          serializer_options.merge!(env[:active_model_serializer_options]) if env[:active_model_serializer_options]
+          ActiveModel::SerializableResource.new(resource, serializer_options).to_json
+        end
+      end
+    end
+  end
+end

lib/grape/helpers/active_model_serializers.rb

@@ -0,0 +1,16 @@
+# Helpers can be included in your grape endpoint as: helpers Grape::Helpers::ActiveModelSerializers
+module Grape
+  module Helpers
+    module ActiveModelSerializers
+      # A convenience method for passing ActiveModelSerializer serializer options
+      #
+      # Example: To include relationships in the response: render(post, include: ['comments'])
+      #
+      # Example: To include pagination meta data: render(posts, meta: { page: posts.page, total_pages: posts.total_pages })
+      def render(resource, active_model_serializer_options = {})
+        env[:active_model_serializer_options] = active_model_serializer_options
+        resource
+      end
+    end
+  end
+end