#11584: make decode_header handle Header objects correctly

This updates b21fdfa0019c, which fixed this bug incorrectly.
diff --git a/Misc/NEWS b/Misc/NEWS
index 170d522..9c3f693 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -25,6 +25,9 @@
 Library
 -------
 
+- Issue #11584: email.header.decode_header no longer fails if the header
+  passed to it is a Header object.
+
 - Issue #11700: mailbox proxy object close methods can now be called multiple
   times without error.