RFC compliance.

Looking at the grammar, I don't see any reference of such a
syntax but there are a lot of similar examples into the RFC so
I guess it is allowed...

fixes #453

modoboa/extensions/webmail/imaputils.py

@@ -138,7 +138,7 @@ class BodyStructure(object):
 class IMAPconnector(object):
     __metaclass__ = ConnectionsManager
 
-    list_base_pattern = r'\((?P<flags>.*?)\) "(?P<delimiter>.*)" "(?P<name>[^"]*)"'
+    list_base_pattern = r'\((?P<flags>.*?)\) "(?P<delimiter>.*)" "?(?P<name>[^"]*)"?'
     list_response_pattern_literal = re.compile(r'\((?P<flags>.*?)\) "(?P<delimiter>.*)" \{(?P<namelen>\d+)\}')
     list_response_pattern = re.compile(list_base_pattern)
     listextended_response_pattern = \