#10790: make append work when output codec is different from input codec

There's still a bug here (the encode call shouldn't use the 'errors'
paramter), but I'll fix that later.
diff --git a/Lib/email/test/test_email.py b/Lib/email/test/test_email.py
index 73ac347..e7fcee3 100644
--- a/Lib/email/test/test_email.py
+++ b/Lib/email/test/test_email.py
@@ -3620,6 +3620,10 @@
         s = 'Subject: =?EUC-KR?B?CSixpLDtKSC/7Liuvsax4iC6uLmwMcijIKHaILzSwd/H0SC8+LCjwLsgv7W/+Mj3I ?='
         raises(errors.HeaderParseError, decode_header, s)
 
+    def test_shift_jis_charset(self):
+        h = Header('文', charset='shift_jis')
+        self.assertEqual(h.encode(), '=?iso-2022-jp?b?GyRCSjgbKEI=?=')
+
 
 
 # Test RFC 2231 header parameters (en/de)coding