fix throttle mechanism

there were two problems:
1) In the throttle case, the callback was being returned, not executed
2) In the case when throttle was not needed, it was neglecting to wrap the result in a Promise.resolve which wasn't causing any problems directly, but it is bad practice to mix return types.

lib/throttler.js

@@ -10,18 +10,18 @@ class Throttler {
         this._timeBetweenRequestsInMilliSeconds = 1 / this.maxRequestsPerSecond * 1000
         this._nextAvailableRequestMillis = getNowMillis()
     }
-
+    
     execute(cb) {
         let nowMillis = getNowMillis()
         if (this.maxRequestsPerSecond === 0) {
-            return cb()
+            return Promise.resolve(cb())
         } else if (nowMillis >= this._nextAvailableRequestMillis) {
             this._nextAvailableRequestMillis = getNowMillis() + this._timeBetweenRequestsInMilliSeconds
-            return cb()
+            return Promise.resolve(cb())
         } else {
             return new Promise((resolve) => {
                 setTimeout(() => {
-                    resolve(cb)
+                    resolve(cb())
                 }, this._nextAvailableRequestMillis - nowMillis)
                 this._nextAvailableRequestMillis += this._timeBetweenRequestsInMilliSeconds
             })

test/acceptance/OperationHelper.specs.js

@@ -25,7 +25,7 @@ describe('OperationHelper', () => {
             let opHelper = new OperationHelper({
                 awsId: config.AWS_ACCESS_KEY_ID,
                 awsSecret: config.AWS_SECRET_ACCESS_KEY,
-                assocId:   config.AWS_ASSOCIATE_ID
+                assocId: config.AWS_ASSOCIATE_ID
             })
 
             return opHelper.execute('ItemSearch', {