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/account_spec.rb

@@ -126,8 +126,8 @@ RSpec.describe Account, type: :model do
       end
 
       it 'sets default avatar, header, avatar_remote_url, and header_remote_url' do
-        expect(account.avatar_remote_url).to eq ''
-        expect(account.header_remote_url).to eq ''
+        expect(account.avatar_remote_url).to eq 'https://remote.test/invalid_avatar'
+        expect(account.header_remote_url).to eq expectation.header_remote_url
         expect(account.avatar_file_name).to  eq nil
         expect(account.header_file_name).to  eq nil
       end