features: writing a configuration file should append if it's already there

features/step_definitions.rb

@@ -93,7 +93,13 @@ end
 #
 
 Given %r{^I have a configuration file with "(.*)" set to "(.*)"$} do |key, value|
-  File.write("_config.yml", "#{key}: #{value}\n")
+  config = if source_dir.join("_config.yml").exist?
+    SafeYAML.load_file(source_dir.join("_config.yml"))
+  else
+    {}
+  end
+  config[key] = value
+  File.write("_config.yml", YAML.dump(config))
 end
 
 #