custom parsers: reduce cognitive complexity

This is according to CodeClimate.

lib/happymapper/item.rb

@@ -118,19 +118,23 @@ def process_node_with_custom_parser(node)
  node.to_s
  end
 
- custom_parser = options[:parser]
+ custom_parser = create_custom_parser(options[:parser])
 
  begin
- if custom_parser.respond_to?(:call)
- custom_parser.call(value)
- else
- constant.send(custom_parser.to_sym, value)
- end
+ custom_parser.call(value)
  rescue StandardError
  nil
  end
  end
 
+ def create_custom_parser(parser)
+ return parser if parser.respond_to?(:call)
+
+ proc { |value|
+ constant.send(parser.to_sym, value)
+ }
+ end
+
  def process_node_with_default_parser(node, parse_options)
  constant.parse(node, options.merge(parse_options))
  end