[FIX] mail: _fields['type'].selection is a callable

This revert partially the rev. f2e4a10e

  File "/home/odoo/src/odoo/8.0/addons/mail/mail_mail.py", line 76, in default_get
      if context and context.get('default_type') and context.get('default_type') not in self._fields['type'].selection:
      TypeError: argument of type 'function' is not iterable