Convert to string when necessary.

fix #671

modoboa/extensions/amavis/views.py

@@ -179,6 +179,8 @@ def viewheaders(request, mail_id):
     content = ""
     for qm in get_connector().get_mail_content(mail_id):
         content += qm.mail_text
+    if type(content) is unicode:
+        content = content.encode("utf-8")
     msg = email.message_from_string(content)
     headers = []
     for name, value in msg.items():