Check `present` validators even when the model is not changed

[hkdobrev]

When you have a loaded, not changed model and execute the check() method, no validator would be executed, even the present one.

If you have some wrong or outdated data in the database and you load the model, if you want to perform the present validators your only option is to use the $force parameter.

See c0b85df

I guess Jam should trigger the present validators every time check() is called.