[JavaScript] Remove default parameters syntax, as it's an ES6 feature (#2848)

* Remove default parameters in JS client, as it's an ES6 feature

* update js petstore samples

Author: f-f

modules/openapi-generator/src/main/resources/Javascript/ApiClient.mustache

@@ -651,7 +651,8 @@
       ];
     };
 
-    exports.getBasePathFromSettings = function(index, variables={}) {
+    exports.getBasePathFromSettings = function(index, variables) {
+        var variables = variables || {};
         var servers = this.hostSettings();
 
         // check array index out of bound

samples/client/petstore/javascript-promise/src/ApiClient.js

@@ -598,7 +598,8 @@
       ];
     };
 
-    exports.getBasePathFromSettings = function(index, variables={}) {
+    exports.getBasePathFromSettings = function(index, variables) {
+        var variables = variables || {};
         var servers = this.hostSettings();
 
         // check array index out of bound

samples/client/petstore/javascript/src/ApiClient.js

@@ -609,7 +609,8 @@
       ];
     };
 
-    exports.getBasePathFromSettings = function(index, variables={}) {
+    exports.getBasePathFromSettings = function(index, variables) {
+        var variables = variables || {};
         var servers = this.hostSettings();
 
         // check array index out of bound