Spaces in title get stripped by downloader, causing errors

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Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2019.03.09. If it's not, read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2019.03.09

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• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
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• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
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If the purpose of this issue is a bug report, site support request or you are not completely sure provide the full verbose output as follows:

Add the -v flag to your command line you run youtube-dl with (youtube-dl -v <your command line>), copy the whole output and insert it here. It should look similar to one below (replace it with your log inserted between triple ```):

[debug] System config: []
[debug] User config: ['--external-downloader', 'aria2c', '--external-downloader-args', '--dir . -j 5 -x 5 -s 5 -c -k 1MB', '--format', 'best[filesize<300MB]/best']
[debug] Custom config: []
[debug] Command-line args: ['-v', 'agZ0xISi40E']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.03.09
[debug] Python version 3.7.2 (CPython) - Linux-5.0.0-arch1-1-ARCH-x86_64-with-arch
[debug] exe versions: ffmpeg 4.1.1, ffprobe 4.1.1
[debug] Proxy map: {}
[youtube] agZ0xISi40E: Downloading webpage
[youtube] agZ0xISi40E: Downloading video info webpage
[debug] Invoking downloader on 'https://r2---sn-uxa15gpou-ua8e.googlevideo.com/videoplayback?mm=31%2C29&mn=sn-uxa15gpou-ua8e%2Csn-4g5ednee&dur=343.074&id=o-AP-fbfCKxvLmhEla-XgdD-1yf7cLsNUkDgXgpKKvIjMx&source=youtube&mv=m&ms=au%2Crdu&fvip=4&key=yt6&ip=141.226.82.15&lmt=1552232510083300&pcm2cms=yes&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpcm2cms%2Cpl%2Cratebypass%2Crequiressl%2Csource%2Cexpire&ipbits=0&ratebypass=yes&txp=4531432&requiressl=yes&c=WEB&initcwndbps=122500&signature=8F39EEABADC02EE40D82941C9FD88CA4A8E13912.88A5E21E25BD600AA7106EC5EE32FB932D9390B0&mime=video%2Fmp4&ei=hGWFXJeCA8im1gLO1ZzIDg&mt=1552246025&pl=24&gir=yes&clen=12989551&itag=18&expire=1552267748'
[download] Destination:  The EU's 'SECRET' Brexit Negotiation EXPOSED 🙄-agZ0xISi40E.mp4
[debug] aria2c command line: aria2c -c --dir . -j 5 -x 5 -s 5 -c -k 1MB --out ' The EU'"'"'s '"'"'SECRET'"'"' Brexit Negotiation EXPOSED 🙄-agZ0xISi40E.mp4.part' --header 'User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:59.0) Gecko/20100101 Firefox/59.0' --header 'Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7' --header 'Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8' --header 'Accept-Encoding: gzip, deflate' --header 'Accept-Language: en-us,en;q=0.5' --check-certificate=true -- 'https://r2---sn-uxa15gpou-ua8e.googlevideo.com/videoplayback?mm=31%2C29&mn=sn-uxa15gpou-ua8e%2Csn-4g5ednee&dur=343.074&id=o-AP-fbfCKxvLmhEla-XgdD-1yf7cLsNUkDgXgpKKvIjMx&source=youtube&mv=m&ms=au%2Crdu&fvip=4&key=yt6&ip=141.226.82.15&lmt=1552232510083300&pcm2cms=yes&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpcm2cms%2Cpl%2Cratebypass%2Crequiressl%2Csource%2Cexpire&ipbits=0&ratebypass=yes&txp=4531432&requiressl=yes&c=WEB&initcwndbps=122500&signature=8F39EEABADC02EE40D82941C9FD88CA4A8E13912.88A5E21E25BD600AA7106EC5EE32FB932D9390B0&mime=video%2Fmp4&ei=hGWFXJeCA8im1gLO1ZzIDg&mt=1552246025&pl=24&gir=yes&clen=12989551&itag=18&expire=1552267748'

03/10 21:33:39 [NOTICE] Downloading 1 item(s)

03/10 21:33:39 [NOTICE] GID#4bc289ee2f73f0e0 - Download has already completed: ./The EU's 'SECRET' Brexit Negotiation EXPOSED 🙄-agZ0xISi40E.mp4.part

03/10 21:33:39 [NOTICE] Download complete: ./The EU's 'SECRET' Brexit Negotiation EXPOSED 🙄-agZ0xISi40E.mp4.part

Download Results:
gid   |stat|avg speed  |path/URI
======+====+===========+=======================================================
4bc289|OK  |       0B/s|./The EU's 'SECRET' Brexit Negotiation EXPOSED 🙄-agZ0xISi40E.mp4.part

Status Legend:
(OK):download completed.
ERROR: unable to download video data: [Errno 2] No such file or directory: " The EU's 'SECRET' Brexit Negotiation EXPOSED 🙄-agZ0xISi40E.mp4.part"
Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/youtube_dl/YoutubeDL.py", line 1913, in process_info
    success = dl(filename, info_dict)
  File "/usr/lib/python3.7/site-packages/youtube_dl/YoutubeDL.py", line 1852, in dl
    return fd.download(name, info)
  File "/usr/lib/python3.7/site-packages/youtube_dl/downloader/common.py", line 364, in download
    return self.real_download(filename, info_dict)
  File "/usr/lib/python3.7/site-packages/youtube_dl/downloader/external.py", line 52, in real_download
    fsize = os.path.getsize(encodeFilename(tmpfilename))
  File "/usr/lib/python3.7/genericpath.py", line 50, in getsize
    return os.stat(filename).st_size
FileNotFoundError: [Errno 2] No such file or directory: " The EU's 'SECRET' Brexit Negotiation EXPOSED 🙄-agZ0xISi40E.mp4.part"

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Description of your issue, suggested solution and other information

Judging by the output, the issue seems to be that aria2c strips the whitespace at the beginning of the file name even when this is explicitly requested. Raised the issue upstream as aria2/aria2#1373,
but I'm noting this here in case someone else runs into this issue.
The problem can be worked around by explicitly renaming the intermediate file to what youtube-dl
expects.

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aria2 are declining to fix the bug, see aria2/aria2#1373 -- they claim
leading/trailing whitespace in filenames is a misfeature.
Argument about whether they should support this behaviour aside, their argument
is fair in that sanitize_filename should also strip whitespace.
Then the default output template wouldn't be sensitive to stripping and anyone
who inserted whitespace in manually would be getting what they deserved.

Thoughts? Implementation would be simply a matter of replacing strip('_') by
strip(' _') in sanitize_filename.