title: Rails: get #beginning_of_day in time zone
body: <p>I have a default time zone setup for the rails application.
  And an instance of the Date object.</p>
  
  <p>How can I get make Date#beginning_of_day to return the beginning of the day in the specified time zone, but not my local timezone.</p>
  
  <p>Is there any other method to get beginning of the day time in the specified timezone for the given date?</p>
  
  <pre><code>date = Date.new(2014,10,29)
  
  zone = ActiveSupport::TimeZone.new('CET')
  date.foo(zone) # should return "Wed, 29 Oct 2014 00:00:00 CET +01:00"
  
  zone = ActiveSupport::TimeZone.new('UTC')
  date.foo(zone) # should return "Wed, 29 Oct 2014 00:00:00 UTC +00:00"
  </code></pre>