Implement detection of modified fields

[romaninsh]

Either Dirty or comparing with the original values. Not sure which one yet

Dirty: will fail if same value is written back.

Original: need to store original value and needlessly compare.

Perhaps a combination can be used, e.g. copy-on-modify.

$m['name'] = 'John';
var_dump($m->saveAction());

^^ the query should only update 'name' and not 'surname'

[romaninsh]

This was fixed in #21