Fixed #18902 -- Made force_bytes properly handle exception input

Thanks Aymeric Augustin for the report and the initial patch.

tests/regressiontests/utils/encoding.py

@@ -0,0 +1,17 @@
+# -*- encoding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.utils import unittest
+from django.utils.encoding import force_bytes
+
+
+class TestEncodingUtils(unittest.TestCase):
+    def test_force_bytes_exception(self):
+        """
+        Test that force_bytes knows how to convert to bytes an exception
+        containing non-ASCII characters in its args.
+        """
+        error_msg = "This is an exception, voilà"
+        exc = ValueError(error_msg)
+        result = force_bytes(exc)
+        self.assertEqual(result, error_msg.encode('utf-8'))