code: def soap_header_handler(auth_handler, version, header_ns, default_ns)
        auth_method = @config.read('authentication.method', :OAUTH2)
        handler_class = case auth_method
          when :OAUTH2, :OAUTH2_SERVICE_ACCOUNT
            AdsCommon::SavonHeaders::OAuthHeaderHandler
          else
            raise AdsCommon::Errors::AuthError,
                "Unknown auth method: %s" % auth_method
          end
        return handler_class.new(@credential_handler, auth_handler, header_ns,
                                    default_ns, version)
      end
docs: Retrieve correct soap_header_handler.
  
   Args:
   - auth_handler: instance of an AdsCommon::Auth::BaseHandler subclass to
     handle authentication
   - version: intended API version
   - header_ns: header namespace
   - default_ns: default namespace
  
   Returns:
   - SOAP header handler