Fix Persistence#touch for aliased fields

Before:

```ruby
class A
  include Mongoid::Document

  field :at, as: :aliased_timestamp, type: Time
end

a = A.create
a.touch(:aliased_timestamp)
a.reload.aliased_timestamp # => nil
```

After:

```ruby
class A
  include Mongoid::Document

  field :at, as: :aliased_timestamp, type: Time
end

a = A.create
a.touch(:aliased_timestamp)
a.reload.aliased_timestamp # => Time
```