Fix exception construction in util.command_output()

`cmd` being a byte string array, it should be joined by b" " and not
u" ".

beets/util/__init__.py

@@ -664,7 +664,7 @@ def command_output(cmd, shell=False):
     if proc.returncode:
         raise subprocess.CalledProcessError(
             returncode=proc.returncode,
-            cmd=' '.join(cmd),
+            cmd=b' '.join(cmd),
         )
     return stdout
 

test/test_util.py

@@ -19,8 +19,9 @@ from __future__ import (division, absolute_import, print_function,
 import sys
 import re
 import os
+import subprocess
 
-from mock import patch
+from mock import patch, Mock
 
 from test._common import unittest
 from test import _common
@@ -102,6 +103,19 @@ class UtilTest(unittest.TestCase):
             ])
         self.assertEqual(p, u'foo/_/bar')
 
+    @patch('beets.util.subprocess.Popen')
+    def test_command_output(self, mock_popen):
+        def popen_fail(*args, **kwargs):
+            m = Mock(returncode=1)
+            m.communicate.return_value = None, None
+            return m
+
+        mock_popen.side_effect = popen_fail
+        with self.assertRaises(subprocess.CalledProcessError) as exc_context:
+            util.command_output([b"taga", b"\xc3\xa9"])
+        self.assertEquals(exc_context.exception.returncode, 1)
+        self.assertEquals(exc_context.exception.cmd, b"taga \xc3\xa9")
+
 
 class PathConversionTest(_common.TestCase):
     def test_syspath_windows_format(self):