* allow the relationship's serializer key to be a Proc
* fixes
* specifically test the relationship name and the resource type
* support object blocks without a serializer defined
* stop validation gracefully if the relationship is polymorphic
* improve performance by using instance variables instead of accessor methods
* force initialization up front to avoid the iterative calls
* serializer procs should act like polymorphic when determining the id_method_name
* specs for serializer procs
* updated with more details and examples for relationship serializer options
* adjust specs to define the serializers
* avoid extra method calls for performance
* name change
* one less function call for better performance
* do not require lazy loaded relationships to resolve the serializer
* give polymorphic precedence for backwards compatibility
* move serializer inference into ObjectSerializer to allow for overriding
* move method for better diff
* fix race condition in multi-threaded environments
