[twitch] Don't pollute default headers dict

diff --git a/youtube_dl/extractor/twitch.py b/youtube_dl/extractor/twitch.py
index 26661fdf635c2891fad68e695595e45b1aad0e51..401615683dc9b22e4110a6db06dd959c524d96f9 100644 (file)
--- a/youtube_dl/extractor/twitch.py
+++ b/youtube_dl/extractor/twitch.py
@@ -51,7 +51,9 @@ class TwitchBaseIE(InfoExtractor):
                 expected=True)
 
     def _call_api(self, path, item_id, *args, **kwargs):
-        kwargs.setdefault('headers', {})['Client-ID'] = self._CLIENT_ID
+        headers = kwargs.get('headers', {}).copy()
+        headers['Client-ID'] = self._CLIENT_ID
+        kwargs['headers'] = headers
         response = self._download_json(
             '%s/%s' % (self._API_BASE, path), item_id,
             *args, **compat_kwargs(kwargs))