Handle case when no encoding is detected

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@21112 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5/Document/EmailDocument.py

@@ -265,13 +265,13 @@ class EmailDocument(File, TextDocument):
     for part in self._getMessage().walk():
       if part.get_content_type() == 'text/plain' and not text_result and not part.is_multipart():
         part_encoding = part.get_content_charset()
-        if part_encoding != 'utf-8':
+        if part_encoding not in (None, 'utf-8',):
           text_result = part.get_payload(decode=1).decode(part_encoding).encode('utf-8')
         else:
           text_result = part.get_payload(decode=1)
       elif part.get_content_type() == 'text/html' and not html_result and not part.is_multipart():
         part_encoding = part.get_content_charset()
-        if part_encoding != 'utf-8':
+        if part_encoding not in (None, 'utf-8',):
           return part.get_payload(decode=1).decode(part_encoding).encode('utf-8')
         return part.get_payload(decode=1)
     return text_result