add 'browser' option (#1117)

- change default user agent to Firefox ESR 78 on Windows 10
- remove 'ciphers' option

gallery_dl/downloader/http.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2014-2020 Mike Fährmann
+# Copyright 2014-2021 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
@@ -94,7 +94,7 @@ class HttpDownloader(DownloaderBase):
                 time.sleep(tries)
 
             tries += 1
-            headers = {}
+            headers = {"Accept": "*/*"}
             file_header = None
 
             # check for .part file