#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/Misc/NEWS b/Misc/NEWS
index 5dd389c..7ce7445 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -30,6 +30,9 @@
 Library
 -------
 
+- Issue #10790: email.header.Header.append's charset logic now works correctly
+  for charsets whose output codec is different from its input codec.
+
 - Issue #10819: SocketIO.name property returns -1 when its closed, instead of
   raising a ValueError, to fix repr().