Fix some review comments

Author: ningyougang

core/controller/src/main/resources/apiv1swagger.json

@@ -1915,7 +1915,7 @@
           "items": {
             "type": "string"
           },
-          "description": "del annotations on the item"
+          "description": "The list of annotations to be deleted from the item"
         },
         "parameters": {
           "type": "array",

core/controller/src/main/scala/org/apache/openwhisk/core/controller/Actions.scala

@@ -537,13 +537,12 @@ trait WhiskActionsApi extends WhiskCollectionAPI with PostActionActivation with
 
     val exec = content.exec getOrElse action.exec
 
-    var newAnnotations = action.annotations
-    content.delAnnotations.map { annotationArray =>
-      annotationArray.foreach { annotation =>
-        newAnnotations -= annotation
+    val newAnnotations = content.delAnnotations
+      .map { annotationArray =>
+        annotationArray.foldRight(action.annotations)((a: String, b: Parameters) => b - a)
       }
-    }
-    newAnnotations = newAnnotations ++ content.annotations
+      .map(_ ++ content.annotations)
+      .getOrElse(action.annotations ++ content.annotations)
 
     WhiskAction(
       action.namespace,

tests/src/test/scala/org/apache/openwhisk/core/cli/test/WskRestBasicUsageTests.scala

@@ -131,7 +131,7 @@ class WskRestBasicUsageTests extends TestHelpers with WskTestHelpers with WskAct
       }
   }
 
-  it should "update an action via passing delAnnotations" in withAssetCleaner(wskprops) { (wp, assetHelper) =>
+  it should "delete the given annotations using delAnnotations" in withAssetCleaner(wskprops) { (wp, assetHelper) =>
     val name = "hello"
 
     assetHelper.withCleaner(wsk.action, name) { (action, _) =>

tests/src/test/scala/system/basic/WskActionTests.scala

@@ -357,7 +357,7 @@ class WskActionTests extends TestHelpers with WskTestHelpers with JsHelpers with
     }
   }
 
-  it should "not delete previous annotation when update action with new annotation" in withAssetCleaner(wskprops) {
+  it should "not delete existing annotations when updating action with new annotation" in withAssetCleaner(wskprops) {
     (wp, assetHelper) =>
       val name = "hello"