plugins.stream: fix a default scheme handling for urls
plugins.stream: fix a default scheme handling for urls

Due to a bug introduced in 707df4f27bf144f4e33dc76ffafbcf2fac2519b8 URLs
with scheme (eg. no http) but with a port number would not get http://
appended by default, as urlparse parses anything before the colon as the
scheme.

wlerin/streamlink,chhe/streamlink,bastimeyer/streamlink,mmetak/streamlink,javiercantero/streamlink,beardypig/streamlink,streamlink/streamlink,streamlink/streamlink,melmorabity/streamlink,gravyboat/streamlink,wlerin/streamlink,chhe/streamlink,gravyboat/streamlink,back-to/streamlink,melmorabity/streamlink,mmetak/streamlink,back-to/streamlink,javiercantero/streamlink,bastimeyer/streamlink,beardypig/streamlink