Bug in mail_thread, false calculation of the defaults in auto subscribing

This bug will affect many modules in a tricky way, as mail.thread is inherited by lots of models.

the default value in context shouldn't be considered "extra" values to be added into the final create_value, because:

1. When user changed the default value,  `create_values[key[8:]] = val' will overwrite the user changes.
2. If the `create` method were called from the back-end script instead of front-end, the default_xxx in context won't be in the final created values

`_add_missing_default_values`  should be called here, as those missing default values will be added by create method eventually, and should be considered in auto subscribing and field tracking.