Proper comparison of DateTime#eql? with ActiveSupport::TimeWithZone instance.

fixes #14178

```ruby
  # Previously:
    n = DateTime.now.in_time_zone # => Fri, 28 Feb 2014 22:00:55 UTC +00:00
    n.eql?(n.dup) # => false
  # Now:
    n = DateTime.now.in_time_zone # => Fri, 28 Feb 2014 22:00:55 UTC +00:00
    n.eql?(n.dup) # => true
```