fix youtube plugin verify_age function to handle 301 responses

paczesiowa@desktop ~/projects/get-flash-videos $ ./get_flash_videos --debug www.youtube.com/watch?v=mGG6TlanJWs
1 plugin installed:
- Joemonster.pm
Trying to open plugin ~/.get_flash_videos/plugins/Www.pm
Trying to open plugin ~/.get_flash_videos/plugins/Youtube.pm
Using method 'youtube' plugin version 0.01 for http://www.youtube.com/watch?v=mGG6TlanJWs
Downloading http://www.youtube.com/watch?v=mGG6TlanJWs
-> GET http://www.youtube.com/watch?v=mGG6TlanJWs
<- 301 text/html; charset=utf-8 (0)
Error: Couldn't download URL: 301 Moved Permanently at /home/paczesiowa/projects/get-flash-videos/lib/FlashVideo/Site/Youtube.pm line 392.

It works fine with this change.