Moves Grape::Endpoint extension into a module

Adding methods into the Grape::Endpoint class that
ActiveModel::Serializers are relying on from ActionController as well as
a few convenience methods for Route and Namespace options. I believe we
can expand on some of the empty methods here to allow for higher (api)
level option setting and defaults. This is a first step in that
direction.

lib/grape-active_model_serializers.rb

@@ -1,4 +1,5 @@
 require 'active_model_serializers'
 require 'grape'
+require 'grape/endpoint_extension'
 require "grape-active_model_serializers/version"
 require "grape-active_model_serializers/formatter"

lib/grape-active_model_serializers/formatter.rb

@@ -1,15 +1,5 @@
 require 'active_record'
 
-# Make the Grape::Endpoint quiack like a controller
-module Grape
-  class Endpoint
-    def default_serializer_options; end
-    def serialization_scope; end
-    def _serialization_scope; end
-    def url_options; end
-  end
-end
-
 module Grape
   module Formatter
     module ActiveModelSerializers
@@ -21,11 +11,7 @@ module Grape
 
         def call(resource, env)
           @endpoint = env["api.endpoint"]
-
-          namespace         = endpoint.settings[:namespace]
-          namespace_options = namespace ? namespace.options : {}
-          route_options     = endpoint.options[:route_options]
-          options           = namespace_options.merge(route_options)
+          options   = endpoint.namespace_options.merge(endpoint.route_options)
 
           serializer = ::ActiveModel::Serializer.build_json(endpoint, resource, options)
 

lib/grape/endpoint_extension.rb

@@ -0,0 +1,25 @@
+#
+# Make the Grape::Endpoint quack like a ActionController
+#
+# This allows us to rely on the ActiveModel::Serializer#build_json method
+# to lookup the approriate serializer.
+#
+module Grape
+  module EndpointExtension
+    def namespace_options
+      settings[:namespace] ? settings[:namespace].options : {}
+    end
+
+    def route_options
+      options[:route_options]
+    end
+
+    def default_serializer_options; end
+    def serialization_scope; end
+    def _serialization_scope; end
+    def url_options; end
+    def controller_name; end
+  end
+
+  Endpoint.send(:include, EndpointExtension)
+end