Add handling for S3 path option being a proc

lib/paperclip/storage/s3.rb

@@ -158,7 +158,7 @@ module Paperclip
           end
 
           unless @options[:url].to_s.match(/\A:s3.*url\Z/) || @options[:url] == ":asset_host"
-            @options[:path] = @options[:path].gsub(/:url/, @options[:url]).gsub(/\A:rails_root\/public\/system/, '')
+            @options[:path] = path_option.gsub(/:url/, @options[:url]).gsub(/\A:rails_root\/public\/system/, '')
             @options[:url]  = ":s3_path_url"
           end
           @options[:url] = @options[:url].inspect if @options[:url].is_a?(Symbol)

spec/paperclip/storage/s3_spec.rb

@@ -1474,6 +1474,26 @@ describe Paperclip::Storage::S3 do
     end
   end
 
+  context "path is a proc" do
+    before do
+      rebuild_model storage: :s3,
+        path: ->(attachment) { attachment.instance.attachment_path }
+
+      @dummy = Dummy.new
+      @dummy.class_eval do
+        def attachment_path
+          '/some/dynamic/path'
+        end
+      end
+      @dummy.avatar = stringy_file
+    end
+
+    it "returns a correct path" do
+      assert_match '/some/dynamic/path', @dummy.avatar.path
+    end
+  end
+
+
   private
 
   def rails_env(env)