Fix multiple setter calls bug

Before this pull request:

```ruby
@event.started_at # => nil

@event.started = true
@event.started_at # => 2014-01-15 15:13:00

@event.started = true
@event.started_at # => 2014-01-15 15:13:01

@event.started = true
@event.started_at # => 2014-01-15 15:13:02
```

After this pull request:

```ruby
@event.started_at # => nil

@event.started = true
@event.started_at # => 2014-01-15 15:13:00

@event.started = true
@event.started_at # => 2014-01-15 15:13:00

@event.started = true
@event.started_at # => 2014-01-15 15:13:00
```