fix Twitch account linking

fixes LIBERAPAYCOM-1Y2

liberapay/elsewhere/_base.py

@@ -500,6 +500,7 @@ class PlatformOAuth2(Platform):
     oauth_default_scope = []
     oauth_email_scope = None
     oauth_follows_scope = ''
+    oauth_include_client_id = None
 
     can_auth_with_client_credentials = None
     use_basic_auth_for_app_session = False
@@ -561,9 +562,10 @@ def get_query_id(self, querystring):
     def handle_auth_callback(self, domain, url, state, unused_arg):
         sess = self.get_auth_session(domain, state=state)
         client_secret = self.get_credentials(domain)[1]
-        sess.fetch_token(self.access_token_url.format(domain=domain),
-                         client_secret=client_secret,
-                         authorization_response=url)
+        sess.fetch_token(
+            self.access_token_url.format(domain=domain), client_secret=client_secret,
+            authorization_response=url, include_client_id=self.oauth_include_client_id,
+        )
         return sess
 
 

liberapay/elsewhere/twitch.py

@@ -14,6 +14,7 @@ class Twitch(PlatformOAuth2):
     # Auth attributes
     # https://dev.twitch.tv/docs/authentication/
     auth_url = 'https://id.twitch.tv/oauth2/authorize'
+    oauth_include_client_id = True
     access_token_url = 'https://id.twitch.tv/oauth2/token'
     can_auth_with_client_credentials = True