[FIX] models.py: prevent erasing of defaut values in create form view

When we create a record, web clients calls by default an onchange on this brand new record.
This onchange erases values that were previously set by default_get and sets them to False.
It seems that we invalidate too many things in onchange implementation
This part is false. `_dirty` must be checked against `newval` in case it is a modified m2o (not the id, the content itself)