[core] Empty format selection string means anything goes.

test/test_YoutubeDL.py

@@ -136,6 +136,11 @@ class TestFormatSelection(unittest.TestCase):
         ]
         info_dict = _make_result(formats)
 
+        ydl = YDL({'format': ''}) # no criteria => anything goes
+        ydl.process_ie_result(info_dict.copy())
+        downloaded = ydl.downloaded_info_dicts[0]
+        self.assertEqual(downloaded['format_id'], '35')
+
         ydl = YDL({'format': '20/47'})
         ydl.process_ie_result(info_dict.copy())
         downloaded = ydl.downloaded_info_dicts[0]

youtube_dl/YoutubeDL.py

@@ -1289,6 +1289,9 @@ class YoutubeDL(object):
                 '{0}\n\t{1}\n\t{2}^'.format(note, format_spec, ' ' * start[1]))
             return SyntaxError(message)
 
+        if not format_spec:
+            format_spec = 'worst'
+
         PICKFIRST = 'PICKFIRST'
         MERGE = 'MERGE'
         SINGLE = 'SINGLE'