Cache custom_generation_options options

This hash only needs to be computed once, and repeatedly building it
results in a lot of memory allocations.

Author: lgebhardt

lib/jsonapi/resource_serializer.rb

@@ -230,7 +230,7 @@ def attributes_hash(source, fetchable_fields)
     end
 
     def custom_generation_options
-      {
+      @_custom_generation_options ||= {
         serializer: self,
         serialization_options: @serialization_options
       }