twitch support fails if channel name contains uppercase characters

[madewokherd]

Checklist

• I'm reporting a broken site support
• I've verified that I'm running youtube-dl version 2020.01.24
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar issues including closed ones

Verbose log

C:\Users\madew>youtube-dl -v https://www.twitch.tv/SpeedrunHypeTV
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'https://www.twitch.tv/SpeedrunHypeTV']
[debug] Encodings: locale cp1252, fs utf-8, out utf-8, pref cp1252
[debug] youtube-dl version 2020.01.24
[debug] Python version 3.6.5 (CPython) - Windows-10-10.0.18362-SP0
[debug] exe versions: ffmpeg 4.0, ffprobe 4.0
[debug] Proxy map: {}
[twitch:stream] SpeedrunHypeTV: Downloading stream JSON
ERROR: SpeedrunHypeTV is offline
Traceback (most recent call last):
  File "c:\users\madew\appdata\local\programs\python\python36\lib\site-packages\youtube_dl\YoutubeDL.py", line 796, in extract_info
    ie_result = ie.extract(url)
  File "c:\users\madew\appdata\local\programs\python\python36\lib\site-packages\youtube_dl\extractor\common.py", line 530, in extract
    ie_result = self._real_extract(url)
  File "c:\users\madew\appdata\local\programs\python\python36\lib\site-packages\youtube_dl\extractor\twitch.py", line 582, in _real_extract
    raise ExtractorError('%s is offline' % channel_id, expected=True)
youtube_dl.utils.ExtractorError: SpeedrunHypeTV is offline

Description

When given a channel url with uppercase characters, the twitch extractor incorrectly reports that the channel is offline. For example:

C:\Users\madew>youtube-dl -s https://www.twitch.tv/SpeedrunHypeTV
[twitch:stream] SpeedrunHypeTV: Downloading stream JSON
ERROR: SpeedrunHypeTV is offline

C:\Users\madew>youtube-dl -s https://www.twitch.tv/speedrunhypetv
[twitch:stream] speedrunhypetv: Downloading stream JSON
[twitch:stream] speedrunhypetv: Downloading channel access token
[twitch:stream] speedrunhypetv: Downloading m3u8 information

The easy work-around is to convert to lowercase, but this behavior is confusing and incorrect because channel url's are case-insensitive in a browser.