Ensure the promise and callback versions of Client#connect always hav…

…e the same behaviour

lib/client.js

@@ -53,16 +53,15 @@ var Client = function (config) {
 
 util.inherits(Client, EventEmitter)
 
-Client.prototype.connect = function (callback) {
+Client.prototype._connect = function (callback) {
   var self = this
   var con = this.connection
   if (this._connecting || this._connected) {
     const err = new Error('Client has already been connected. You cannot reuse a client.')
-    if (callback) {
+    process.nextTick(() => {
       callback(err)
-      return undefined
-    }
-    return Promise.reject(err)
+    })
+    return
   }
   this._connecting = true
 
@@ -220,16 +219,23 @@ Client.prototype.connect = function (callback) {
   con.on('notice', function (msg) {
     self.emit('notice', msg)
   })
+}
+
+Client.prototype.connect = function (callback) {
+  if (callback) {
+    this._connect(callback)
+    return
+  }
 
-  if (!callback) {
-    return new global.Promise((resolve, reject) => {
-      this.once('error', reject)
-      this.once('connect', () => {
-        this.removeListener('error', reject)
+  return new Promise((resolve, reject) => {
+    this._connect((error) => {
+      if (error) {
+        reject(error)
+      } else {
         resolve()
-      })
+      }
     })
-  }
+  })
 }
 
 Client.prototype._attachListeners = function (con) {