Accomodate new extends pattern

jsonapi-serializable switched from prepend to extend. To make this still
work, extend after inheriting.

Author: Lee Richmond

gemfiles/rails_4.gemfile

@@ -3,7 +3,7 @@
 source "https://rubygems.org"
 
 gem "rails", "~> 4.1"
-gem "jsonapi-rails", "~> 0.1.0", :require => "jsonapi/rails"
+gem "jsonapi-rails", "~> 0.2.0", :require => "jsonapi/rails"
 gem "rspec-rails"
 
 group :test do

gemfiles/rails_5.gemfile

@@ -3,7 +3,7 @@
 source "https://rubygems.org"
 
 gem "rails", "~> 5.0"
-gem "jsonapi-rails", "~> 0.1.0", :require => "jsonapi/rails"
+gem "jsonapi-rails", "~> 0.2.0", :require => "jsonapi/rails"
 gem "rspec-rails"
 
 group :test do

lib/jsonapi_compliable/extensions/extra_attribute.rb

@@ -57,6 +57,12 @@ def extra_attribute(name, options = {}, &blk)
 end
 
 JSONAPI::Serializable::Resource.class_eval do
-  prepend JSONAPI::Serializable::Resource::ConditionalFields
+  def self.inherited(klass)
+    super
+    klass.class_eval do
+      extend JSONAPI::Serializable::Resource::ConditionalFields
+    end
+  end
+
   include JsonapiCompliable::Extensions::ExtraAttribute
 end