update JS petstore samples

samples/client/petstore/javascript-es6/src/ApiClient.js

@@ -83,6 +83,12 @@ export default class ApiClient {
         if (typeof window === 'undefined') {
           this.agent = new superagent.agent();
         }
+
+        /*
+         * Allow user to override superagent agent
+         */
+         this.requestAgent = null;
+
     }
 
     /**
@@ -398,6 +404,11 @@ export default class ApiClient {
         // set header parameters
         request.set(this.defaultHeaders).set(this.normalizeParams(headerParams));
 
+        // set requestAgent if it is set by user
+        if (this.requestAgent) {
+          request.agent(this.requestAgent);
+        }
+
         // set request timeout
         request.timeout(this.timeout);