[youtube] Don't call 'unquote_plus' on the video title (fixes #2799)

It's already unquoted after calling 'compat_parse_qs'.
It replaced '+' with spaces, for example in https://www.youtube.com/watch?v=XC0b5YexO-I.

youtube_dl/extractor/youtube.py

@@ -1117,7 +1117,7 @@ class YoutubeIE(YoutubeBaseInfoExtractor, SubtitlesInfoExtractor):
 
         # title
         if 'title' in video_info:
-            video_title = compat_urllib_parse.unquote_plus(video_info['title'][0])
+            video_title = video_info['title'][0]
         else:
             self._downloader.report_warning(u'Unable to extract video title')
             video_title = u'_'