[GenericIE] Fix redirects / [YoutubeUserIE] Fix/Simplify VALID_URL

Fix 1: Program crashs with python3.4 if server returns error 405.
Fix 2: youtube.com always returns error 404 (instead of 405) if HEAD request is sent, so redirects are not working at all.
Fix 3: YoutubeUserIE currently matches youtube.com/watch_videos... URLs which are very common on channel pages. I changed VALID_URL in YoutubeUserIE to only match youtube.com/user/... URLs because all short forms like youtube.com/USERNAME get redirected (Fix 1 and 2) to youtube.com/user/USERNAME. youtube.com/watch_videos... URLs get redirected to something YoutubePlaylistIE can handle so they now work too.

This PR is the same as #2965. I created a new PR because some strange commits are showing up there.