luassert: unknown modifier/assertion: 'has_body'

[lubw7]

In my plugin test case, I try to use assert.response(r).has.body() to assert response baby, but an error occurred.
I want to test http_mock, so I don't appoint plugins for my route, and I use pongo run test case.

local helpers = require "spec.helpers"
local http_mock = require("spec.helpers.http_mock")

local PLUGIN_NAME = "my-plugin"

for _, strategy in helpers.all_strategies() do if strategy ~= "cassandra" then
    describe(PLUGIN_NAME .. ": (access) [#" .. strategy .. "]", function()
        local client

        lazy_setup(function()

            mock, mock_port = assert(http_mock.new(8000, {
                ["/"] = {
                    content = [[
                      ngx.header.content_type = "application/json"
                      ngx.say("hello world")
                      ngx.exit(200)
                    ]],
                }
            }))
            assert(mock:start())

            local bp = helpers.get_db_utils(strategy == "off" and "postgres" or strategy, nil, { PLUGIN_NAME })

            -- Inject a test route. No need to create a service, there is a default
            -- service which will echo the request.

            local service = bp.services:insert {
                name = "sso",
                host = helpers.mock_upstream_host,
                port = 8000,
                protocol = helpers.mock_upstream_protocol,
            }

            local route = bp.routes:insert({
                service = service,
                paths = {'/test'}
            })
            -- add the plugin to test to the route we created

            --bp.plugins:insert {
            --    name = PLUGIN_NAME,
            --    route = { id = route1.id },
            --    config = {},
            --}

            -- start kong
            assert(helpers.start_kong({
                -- set the strategy
                database   = strategy,
                -- use the custom test template to create a local mock server
                nginx_conf = "spec/fixtures/custom_nginx.template",
                -- make sure our plugin gets loaded
                plugins = "bundled," .. PLUGIN_NAME,
                -- write & load declarative config, only if 'strategy=off'
                declarative_config = strategy == "off" and helpers.make_yaml_file() or nil,
            }))
        end)

        lazy_teardown(function()
            helpers.stop_kong(nil, true)
            if mock then
                mock:stop()
            end
        end)

        before_each(function()
            client = helpers.proxy_client(nil, 8000, "localhost")
        end)

        after_each(function()
            if client then client:close() end
        end)

        describe("request", function()
            it("gets a 'hello-world' header", function()
                local r = client:get("/request", {
                    headers = {
                        host = "test1.com"
                    }
                })
                -- validate that the request succeeded, response status 200
                assert.response(r).has.status(200)
                assert.response(r).has.body("hello world")
            end)
        end)


    end)

end end

When I run command pongo run

Error → /kong-plugin/spec/my-plugin/04-test_mock_spec.lua @ 79
my-plugin: (access) [#off] request gets a 'hello-world' header
/kong-plugin/spec/my-plugin/04-test_mock_spec.lua:88: luassert: unknown modifier/assertion: 'has_body'

But in https://github.com/Kong/kong/blob/master/spec/helpers/http_mock.lua , demo code use assert.response(r).has.body("hello world").