Allow includes to follow namespacing

(cherry picked from commit 2f46f58)

Author: nagirrab

lib/jsonapi/request_parser.rb

@@ -201,7 +201,7 @@ def check_include(resource_klass, include_parts)
       relationship = resource_klass._relationship(relationship_name)
       if relationship && format_key(relationship_name) == include_parts.first
         unless include_parts.last.empty?
-          check_include(Resource.resource_for(@resource_klass.module_path + relationship.class_name.to_s.underscore), include_parts.last.partition('.'))
+          check_include(Resource.resource_for(resource_klass.module_path + relationship.class_name.to_s.underscore), include_parts.last.partition('.'))
         end
       else
         @errors.concat(JSONAPI::Exceptions::InvalidInclude.new(format_key(resource_klass._type),