title: determining viability of routes generated with polymorphic_url
body: <p>I've got a generic partial that includes a link if the object is linkable.</p>
  
  <p>The URL is generated with polymorphic_url.  If polymorphic_url raises NoMethodError, then I don't include the link.</p>
  
  <p>This mostly works great, except where a resource has an update or destroy action but not a show action.  In that case polymorphic_url generates a valid URL, it's just not valid with GET.</p>
  
  <p>Is there any way to restrict polymorphic_url to a specific request method, or a nice way to determine if the returned URL is valid for GET?</p>