allow set custom value on recover

[Wolfer]

When paranoid column set as NOT NULL, recovery fail with error(e.g. "Mysql2::Error: Column 'is_del' cannot be null" ).
This PR allow set custom value with option "recovery_value" for acts_as_paranoid

[rob-murray]

If the column_type is boolean maybe the recovery_value should be set to false by default too? Or a method similar to delete_now_value for default values.

[Wolfer]

Not all need the recovery_value set as false for boolean type. I tried to minimum change the default behavior of gem.