[generic] Support double slash URLs (Fixes #1309)

youtube_dl/extractor/generic.py

@@ -7,12 +7,14 @@ from .common import InfoExtractor
 from ..utils import (
     compat_urllib_error,
     compat_urllib_parse,
+    compat_urllib_parse_urlparse,
     compat_urllib_request,
 
     ExtractorError,
 )
 from .brightcove import BrightcoveIE
 
+
 class GenericIE(InfoExtractor):
     IE_DESC = u'Generic downloader that works on some sites'
     _VALID_URL = r'.*'
@@ -161,6 +163,8 @@ class GenericIE(InfoExtractor):
             raise ExtractorError(u'Invalid URL: %s' % url)
 
         video_url = compat_urllib_parse.unquote(mobj.group(1))
+        if video_url.startswith('//'):
+            video_url = compat_urllib_parse_urlparse(url).scheme + ':' + video_url
         video_id = os.path.basename(video_url)
 
         # here's a fun little line of code for you: