Log Message:
youtube-dl: Update to 20190520

Changes:
2019.05.20
----------
Core
+ [extractor/common] Move workaround for applying first Set-Cookie header
  into a separate _apply_first_set_cookie_header method

Extractors
* [safari] Fix authentication (#21090)
* [vk] Use _apply_first_set_cookie_header
* [vrt] Fix extraction (#20527)
+ [canvas] Add support for vrtnieuws and sporza site ids and extract
  AES HLS formats
+ [vrv] Extract captions (#19238)
* [tele5] Improve video id extraction
* [tele5] Relax URL regular expression (#21020, #21063)
* [svtplay] Update API URL (#21075)
+ [yahoo:gyao] Add X-User-Agent header to dam proxy requests (#21071)