[FIX] web: don't discard x2many rows coming from default_get

Problem occured in the MRP Production wizard (with a consumed
product tracked by serial number). The opened record is a new
record, and its one2many has 2 default rows that are prefilled.
If the user edited one of the quantities in those rows, and set
an invalid value instead (e.g. a letter), the row was removed
without notice when it was focused out. This was because the
model wasn't aware that a change has been done on the record,
and as the record was flagged as new, it was simply abandonned.