add HttpRequestOptions.dontFollowRedirects (#3473)

This implements the option to have http.request() not follow server's 3xx redirection replies
but instead return the exact redirection code and headers.

* on iOS, it uses a different NSURLSession instance for non-following request,
  with a NSURLSessionTaskDelegate implementing URLSessinTaskWillPerformHTTPRedirection...

* on Android, it just passes the option on to org.nativescript.widgets.Async.Http;
  so this requires the respective commit in tns-core-modules-widgets to work

tns-platform-declarations/android/org.nativescript.widgets.d.ts

@@ -30,6 +30,7 @@
                         public timeout: number;
                         public screenWidth: number;
                         public screenHeight: number;
+                        public dontFollowRedirects: boolean;
                     }
 
                     export class RequestResult {