[vk] Fix authentication on python3

youtube_dl/extractor/vk.py

@@ -52,8 +52,9 @@ def _login(self):
         # what actually happens.
         # We will workaround this VK issue by resetting the remixlhk cookie to
         # the first one manually.
-        cookies = url_handle.headers.get('Set-Cookie')
-        if cookies:
+        for header, cookies in url_handle.headers.items():
+            if header.lower() != 'set-cookie':
+                continue
             if sys.version_info[0] >= 3:
                 cookies = cookies.encode('iso-8859-1')
             cookies = cookies.decode('utf-8')
@@ -61,6 +62,7 @@ def _login(self):
             if remixlhk:
                 value, domain = remixlhk.groups()
                 self._set_cookie(domain, 'remixlhk', value)
+                break
 
         login_page = self._download_webpage(
             'https://login.vk.com/?act=login', None,