UPGRADE FROM 2.2 to 2.3
=======================
### Form
* Although this was not officially supported nor documented, it was possible to
set the option "validation_groups" to false, resulting in the group "Default"
being validated. Now, if you set "validation_groups" to false, the validation
of a form will be skipped (except for a few integrity checks on the form).
If you want to validate a form in group "Default", you should either
explicitly set "validation_groups" to "Default" or alternatively set it to
null.
Before:
```
// equivalent notations for validating in group "Default"
"validation_groups" => null
"validation_groups" => "Default"
"validation_groups" => false
// notation for skipping validation
"validation_groups" => array()
After:
// equivalent notations for skipping validation