Fix UnicodeEncodeError when string comparison with unicode has failed.

testing/test_assertion.py

@@ -816,3 +816,12 @@ def test_assert_indirect_tuple_no_warning(testdir):
     result = testdir.runpytest('-rw')
     output = '\n'.join(result.stdout.lines)
     assert 'WR1' not in output
+
+def test_assert_with_unicode(monkeypatch, testdir):
+    testdir.makepyfile(u"""
+        # -*- coding: utf-8 -*-
+        def test_unicode():
+            assert u'유니코드' == u'Unicode'
+    """)
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines(['*AssertionError*'])