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.
to_s method of HTTP::Response keeps blocking while it receives the whole
content, no matter how it is big. This means it may waste time to receive
unacceptably large files. It may also consume memory and disk in the
process. This solves the inefficency by checking response length while
receiving.
