Fix attachment not being re-downloaded even if file is not stored (#12125)

Change the behaviour of remotable concern. Previously, it would skip
downloading an attachment if the stored remote URL is identical to
the new one. Now it would not be skipped if the attachment is not
actually currently stored by Paperclip.

spec/models/concerns/remotable_spec.rb

@@ -18,6 +18,8 @@ RSpec.describe Remotable do
 
     def hoge=(arg); end
 
+    def hoge_file_name; end
+
     def hoge_file_name=(arg); end
 
     def has_attribute?(arg); end
@@ -109,12 +111,21 @@ RSpec.describe Remotable do
       end
 
       context 'foo[attribute_name] == url' do
-        it 'makes no request' do
+        it 'makes no request if file is saved' do
           allow(foo).to receive(:[]).with(attribute_name).and_return(url)
+          allow(foo).to receive(:hoge_file_name).and_return('foo.jpg')
 
           foo.hoge_remote_url = url
           expect(request).not_to have_been_requested
         end
+
+        it 'makes request if file is not saved' do
+          allow(foo).to receive(:[]).with(attribute_name).and_return(url)
+          allow(foo).to receive(:hoge_file_name).and_return(nil)
+
+          foo.hoge_remote_url = url
+          expect(request).to have_been_requested
+        end
       end
 
       context "scheme is https, parsed_url.host isn't empty, and foo[attribute_name] != url" do