Added normalize_RFC_2822_date() to be-mbox-to-xml.

Added normalize_RFC_2822_date() to be-mbox-to-xml.

The
  if new_fields[u'date'] != None:
bit avoids attemting to normalize missing dates (which fails).  You
get messages with missing dates when comment_message_to_xml() is
called recursively for multipart messages.

Also fixed some unicode keys (['X'] -> [u'X']) for consistency.