OnDeleteTransactor has no effect when context is obtained with DbProxy.GetContext

[miyconst]

Reported by @Dhaigvip.

A simple scenario to reproduce the issue:

public void TriggerCallbackWhenDeleted()
{
    var transactor = CreateServices(
        s => s.Decorate<ITransactor, OnDeleteTransactor>())
        .GetRequiredService<ITransactor>();

    var wasDeleted = transactor.Transact(db =>
    {
        var p = db.Insert<Person>();

        var deleted = false;
        p.WhenDeleted(p => deleted = true);

        db.Delete(p);

        return deleted;
    });

    Assert.True(wasDeleted);

    wasDeleted = transactor.Transact(db =>
    {
        var p = db.Insert<Person>();

        var deleted = false;
        p.WhenDeleted(p => deleted = true);

        // The WhenDeleted method is not called,
        // because DbProxy.GetContext returns the original not decorated `IDatabaseContext`.
        db = DbProxy.GetContext(p);
        db.Delete(p);

        return deleted;
    });

    // This fails.
    Assert.True(wasDeleted);
}

[miyconst]

This limitation can be easily worked around with the following code:

public class OnDeleteTransactor : TransactorBase<object>
{
    /* ... */

    public static IDatabaseContext GetContext(object proxy)
    {
        IDatabaseContext db = DbProxy.GetContext(proxy);
        return new OnDeleteContext(db);
    }

    /* ... */
}

Later on, the OnDeleteTransactor.GetContext method should be used instead of DbProxy.GetContext.

[miyconst]

cc @per-samuelsson

[eriksunsol]

I can confirm that the proposed solution seems to work as expected.