Add contains method for ActiveValue

Motivation

Sometimes it is necessary to compare a value in a model that is already the active model. Now this is done somewhat inconveniently, you need to use !is_not_set() and as_ref(). Which makes the code a bit more complicated.

Proposed Solutions

let id_set = ActiveValue::Set(1);
let id_unchanged = ActiveValue::Unchanged(1);
let id_not_set = ActiveValue::<i32>::NotSet;

id_set.contains(&1); // true
id_unchanged.contains(&1); // true
id_not_set.contains(&1); // false

Current Workarounds

let id_set = ActiveValue::Set(1);

!id_set.is_not_set() && id_set.as_ref() == &1