Update https.ios.ts

Author: kefahB

src/https.ios.ts

@@ -137,7 +137,8 @@ export function request(opts: Https.HttpsRequestOptions): Promise<Https.HttpsRes
   return new Promise((resolve, reject) => {
     try {
       const manager = AFHTTPSessionManager.alloc().initWithBaseURL(NSURL.URLWithString(opts.url));
-      let contentType = opts.headers['Content-Type'].substring(0, 16);
+      
+      let contentType = (<string>opts.headers['Content-Type']).substring(0, 16);
       if (opts.headers && contentType === 'application/json') {
           manager.requestSerializer = AFJSONRequestSerializer.serializer();
           manager.responseSerializer = AFJSONResponseSerializer.serializerWithReadingOptions(NSJSONReadingOptions.AllowFragments);