Fix the method signature of Interpolations.hash to accept no argument

This will make sure Interpolations is comply with Ruby's Object#hash

Closes #569, Closes #603

lib/paperclip/interpolations.rb

@@ -136,8 +136,12 @@ module Paperclip
 
     # Returns a the attachment hash.  See Paperclip::Attachment#hash for
     # more details.
-    def hash attachment, style_name
-      attachment.hash(style_name)
+    def hash attachment=nil, style_name=nil
+      if attachment && style_name
+        attachment.hash(style_name)
+      else
+        super()
+      end
     end
 
     # Returns the id of the instance in a split path form. e.g. returns

test/interpolations_test.rb

@@ -171,13 +171,20 @@ class InterpolationsTest < Test::Unit::TestCase
     assert_equal seconds_since_epoch, Paperclip::Interpolations.updated_at(attachment, :style)
   end
 
-  should "return hash" do
+  should "return attachment's hash when passing both arguments" do
     attachment = mock
     fake_hash = "a_wicked_secure_hash"
     attachment.expects(:hash).returns(fake_hash)
     assert_equal fake_hash, Paperclip::Interpolations.hash(attachment, :style)
   end
 
+  should "return Object#hash when passing no argument" do
+    attachment = mock
+    fake_hash = "a_wicked_secure_hash"
+    attachment.expects(:hash).never.returns(fake_hash)
+    assert_not_equal fake_hash, Paperclip::Interpolations.hash
+  end
+
   should "call all expected interpolations with the given arguments" do
     Paperclip::Interpolations.expects(:id).with(:attachment, :style).returns(1234)
     Paperclip::Interpolations.expects(:attachment).with(:attachment, :style).returns("attachments")