code: def generate_user_agent(extra_ids = [], agent_app = nil)
        agent_app ||= File.basename($0)
        agent_data = extra_ids
        agent_data << 'Common-Ruby/%s' % AdsCommon::ApiConfig::CLIENT_LIB_VERSION
        agent_data << 'GoogleAdsSavon/%s' % GoogleAdsSavon::VERSION
        ruby_engine = defined?(RUBY_ENGINE) ? RUBY_ENGINE : 'ruby'
        agent_data << [ruby_engine, RUBY_VERSION].join('/')
        agent_data << 'HTTPI/%s' % HTTPI::VERSION
        agent_data << HTTPI::Adapter.use.to_s
        agent_data += get_extra_user_agents()
        return '%s (%s)' % [agent_app, agent_data.join(', ')]
      end
docs: Generates string for UserAgent to put into headers.