[nerdcubed] Style fixes
[tmz] Fix test's thumbnail field
[rtp] Fix test's id field
[internetvideoarchive] Update test's duration field
[telecinco] Rename 'episode' group to 'id' in the _VALID_URL regex

MiTeleIE now uses '_match_id'
[dbtv] Make sure the 'id' field is a string
[extractor/common] Update docstring: replace FileDownloader with YoutubeDL