[generic] Allow parsing when first 512 bytes are whitespace

is_html(first_bytes) will fail if the first 512 bytes of the URL are
all whitespace, for some weird reason. Such a case probably is not a
direct video link, the case we're concerned about downloading
inadvertently, since that wouldn't be a valid video binary file
format.

But it's still peculiar, so don't silently ignore it -- print a
warning and continue on.

youtube_dl/extractor/generic.py

@@ -1759,9 +1759,12 @@ class GenericIE(InfoExtractor):
             self._sort_formats(info_dict['formats'])
             return info_dict
 
-        # Maybe it's a direct link to a video?
-        # Be careful not to download the whole thing!
-        if not is_html(first_bytes):
+        if re.match(r'^\s+$', first_bytes):
+            self._downloader.report_warning(
+                'First block is just whitespace? Continuing...')
+        elif not is_html(first_bytes):
+            # Maybe it's a direct link to a video?
+            # Be careful not to download the whole thing!
             self._downloader.report_warning(
                 'URL could be a direct video link, returning it as such.')
             info_dict.update({