youtube: Simplify video ratings handling code
youtube: Simplify video ratings handling code

This simplifies the code and fixes a problem where if both
region_restriction_allowed and region_restriction_blocked were not
spcified, the GDataYouTubeVideo would behave as if restricted for all
regions, which is most likely not the intention.

pwithnall/libgdata,pwithnall/libgdata,GNOME/libgdata