#1368247: make set_charset/MIMEText automatically encode unicode _payload.

Fixes (mysterious, to the end user) UnicodeErrors when using utf-8 as
the charset and unicode as the _text argument.  Also makes the way in
which unicode gets encoded to quoted printable for other charsets more
sane (it only worked by accident previously).  The _payload now is encoded
to the charset.output_charset if it is unicode.
diff --git a/Lib/email/message.py b/Lib/email/message.py
index 993a1ac..08423cd 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -256,6 +256,8 @@
                             charset=charset.get_output_charset())
         else:
             self.set_param('charset', charset.get_output_charset())
+        if isinstance(self._payload, unicode):
+            self._payload = self._payload.encode(charset.output_charset)
         if str(charset) != charset.get_output_charset():
             self._payload = charset.body_encode(self._payload)
         if 'Content-Transfer-Encoding' not in self: