mongoid/mongoid — pull request #2884 by Nielsomat (2013-03-11 11:36:42+00:00)
Title: 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
```