issue2551092 - fix crash due to use of string not bytes under py3

Align roundup.anypy.email_.decode_header which is a copy of the stdlib
email.header.decode_header by adding conversion of str to bytes for
py3.

rouilj: I was unable to figure out how to generate a test case for
this. Existing tests that use utf8 encoded subject line pass and in all
cases including the ones where I tried to craft a failing subject,
always were bytes and not strings.

Author: rouilj

CHANGES.txt

@@ -39,6 +39,9 @@ Fixed:
 - issue2551094 - make simplemde handle line breaks the same as the
   backend markdown formmatters. (report: Cedric Krier, patch: Christof
   Meerwald) 
+- issue2551092 - fix crash bug by aligning
+  roundup.anypy.email_.decode_header with stdlib email.header and
+  convert string to bytes for python 3. (Cedrick Krier)
 
 Features:
 - issue2550522 - Add 'filter' command to command-line

roundup/anypy/email_.py

@@ -108,8 +108,8 @@ def decode_header(header):
     collapsed = []
     last_word = last_charset = None
     for word, charset in decoded_words:
-        if isinstance(word, str):
-            pass
+        if isinstance(word, str) and bytes != str:
+            word = bytes(word, 'raw-unicode-escape')
         if last_word is None:
             last_word = word
             last_charset = charset