utils.shell_quote: Convert the args to unicode strings

The youtube test video failed with `UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 34: ordinal not in range(128)`, the problem was with the filenames being encoded.

test/test_utils.py

@@ -24,6 +24,8 @@ from youtube_dl.utils import (
     xpath_with_ns,
     smuggle_url,
     unsmuggle_url,
+    shell_quote,
+    encodeFilename,
 )
 
 if sys.version_info < (3, 0):
@@ -170,6 +172,10 @@ class TestUtil(unittest.TestCase):
         self.assertEqual(res_url, url)
         self.assertEqual(res_data, None)
 
+    def test_shell_quote(self):
+        args = ['ffmpeg', '-i', encodeFilename(u'ñ€ß\'.mp4')]
+        self.assertEqual(shell_quote(args), u"""ffmpeg -i 'ñ€ß'"'"'.mp4'""")
+
 
 if __name__ == '__main__':
     unittest.main()