fix: add missing export of UniqueMessageIdProvider and DefaultMessageIdProvider (#1572)

Author: ogis-ksuzuki

lib/connect/index.js

@@ -2,6 +2,8 @@
 
 const MqttClient = require('../client')
 const Store = require('../store')
+const DefaultMessageIdProvider = require('../default-message-id-provider')
+const UniqueMessageIdProvider = require('../unique-message-id-provider')
 const IS_BROWSER = require('../is-browser').IS_BROWSER
 const url = require('url')
 const xtend = require('xtend')
@@ -163,3 +165,5 @@ module.exports = connect
 module.exports.connect = connect
 module.exports.MqttClient = MqttClient
 module.exports.Store = Store
+module.exports.DefaultMessageIdProvider = DefaultMessageIdProvider
+module.exports.UniqueMessageIdProvider = UniqueMessageIdProvider