plugins.cdnbg: simplify and fix iframes without schema
plugins.cdnbg: simplify and fix iframes without schema

beardypig/streamlink,bastimeyer/streamlink,beardypig/streamlink,gravyboat/streamlink,melmorabity/streamlink,streamlink/streamlink,gravyboat/streamlink,melmorabity/streamlink,bastimeyer/streamlink,chhe/streamlink,streamlink/streamlink,chhe/streamlink