Use built-in response.parsed_body for JSON response specs (#31674)

spec/spec_helper.rb

@@ -39,11 +39,7 @@ RSpec.configure do |config|
 end
 
 def body_as_json
-  json_str_to_hash(response.body)
-end
-
-def json_str_to_hash(str)
-  JSON.parse(str, symbolize_names: true)
+  response.parsed_body
 end
 
 def serialized_record_json(record, serializer, adapter: nil)