Cache computed attribute options

Merging the default and the attribute specific options was resulting in
a lot of memory allocations

Author: lgebhardt

lib/jsonapi/basic_resource.rb

@@ -453,6 +453,8 @@ def inherited(subclass)
 
         subclass._routed = false
         subclass._warned_missing_route = false
+
+        subclass._clear_cached_attribute_options
       end
 
       def rebuild_relationships(relationships)
@@ -527,6 +529,8 @@ def attributes(*attrs)
       end
 
       def attribute(attribute_name, options = {})
+        _clear_cached_attribute_options
+
         attr = attribute_name.to_sym
 
         check_reserved_attribute_name(attr)
@@ -826,7 +830,7 @@ def verify_relationship_filter(filter, raw, _context = nil)
 
       # quasi private class methods
       def _attribute_options(attr)
-        default_attribute_options.merge(@_attributes[attr])
+        @_cached_attribute_options[attr] ||= default_attribute_options.merge(@_attributes[attr])
       end
 
       def _attribute_delegated_name(attr)
@@ -1107,6 +1111,10 @@ def register_relationship(name, relationship_object)
         @_relationships[name] = relationship_object
       end
 
+      def _clear_cached_attribute_options
+        @_cached_attribute_options = {}
+      end
+
       private
 
       def check_reserved_resource_name(type, name)