Do not show warning if :class exists in the interpolation URL

This fixes #660

lib/paperclip.rb

@@ -193,7 +193,7 @@ def class_for(class_name)
     def check_for_url_clash(name,url,klass)
       @names_url ||= {}
       default_url = url || Attachment.default_options[:url]
-      if @names_url[name] && @names_url[name][:url] == default_url && @names_url[name][:class] != klass
+      if @names_url[name] && @names_url[name][:url] == default_url && @names_url[name][:class] != klass && @names_url[name][:url] !~ /:class/
         log("Duplicate URL for #{name} with #{default_url}. This will clash with attachment defined in #{@names_url[name][:class]} class")
       end
       @names_url[name] = {:url => default_url, :class => klass}

test/paperclip_test.rb

@@ -86,6 +86,16 @@ class Dummy2 < ActiveRecord::Base
         has_attached_file :blah
       end
     end
+
+    should "not generate warning if attachment is redifined with the same url string but has :class in it" do
+      Paperclip.expects(:log).never
+      Dummy.class_eval do
+        has_attached_file :blah, :url => "/system/:class/:attachment/:id/:style/:filename"
+      end
+      Dummy2.class_eval do
+        has_attached_file :blah, :url => "/system/:class/:attachment/:id/:style/:filename"
+      end
+    end
   end
 
   context "An ActiveRecord model with an 'avatar' attachment" do