More email package fixes.

MIMEApplication() requires a bytes object for its _data, so fix the tests.

We no longer need utils._identity() or utils._bdecode().  The former isn't
used anywhere AFAICT (where's "make test's" lint? <wink>) and the latter is a
kludge that is eliminated by base64.b64encode().

Current status: 5F/5E
diff --git a/Lib/email/test/test_email.py b/Lib/email/test/test_email.py
index ef11e17..15c8325 100644
--- a/Lib/email/test/test_email.py
+++ b/Lib/email/test/test_email.py
@@ -1012,7 +1012,7 @@
 class TestMIMEApplication(unittest.TestCase):
     def test_headers(self):
         eq = self.assertEqual
-        msg = MIMEApplication('\xfa\xfb\xfc\xfd\xfe\xff')
+        msg = MIMEApplication(b'\xfa\xfb\xfc\xfd\xfe\xff')
         eq(msg.get_content_type(), 'application/octet-stream')
         eq(msg['content-transfer-encoding'], 'base64')