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-core-modules/http/http.d.ts

@@ -89,6 +89,11 @@ declare module "http" {
      * Gets or sets the request timeout in milliseconds.
      */
     timeout?: number;
+
+    /**
+     * Gets or sets wether to *not* follow server's redirection responses.
+     */
+    dontFollowRedirects?: boolean;
   }
 
   /**