added possibility to cast any key to any type using any method

lib/couchrest/core/model.rb

@@ -537,13 +537,14 @@ module CouchRest
       self.class.casts.each do |k,v|
         next unless self[k]
         target = v[:as]
+        v[:send] ||= 'new'
         if target.is_a?(Array)
           klass = ::Extlib::Inflection.constantize(target[0])
           self[k] = self[k].collect do |value|
-            klass.new(value)
+            klass.send(v[:send], value)
           end
         else
-          self[k] = ::Extlib::Inflection.constantize(target).new(self[k])
+          self[k] = ::Extlib::Inflection.constantize(target).send(v[:send], self[k])
         end
       end
     end
@@ -554,4 +555,4 @@ module CouchRest
     register_instance_hooks :save, :create, :update, :destroy
 
   end # class Model
-end # module CouchRest
+end # module CouchRest