Closes #678. Closes #450.

If a model has an existing attachment and styles, and later a new attachment is added (not just new styles), "rake paperclip:refresh:missing_styles" would crash with "TypeError: can't convert nil into Array".

#fingerprint now consistently returns nil before and after saving an attachment if the value can't be written to the db.

From https://github.com/pcreux/paperclip/commit/52374958ef83f1641cf008aead27c23b25fb842d

Conflicts:

	lib/paperclip/attachment.rb

This fixes #660

Standardize strip with resy of application.  Without to_s, causes errors like "undefined method `strip' for image/bmp:MIME::Type".

In some case, we seriously don't want to prepend the timestamp with & if there wasn't a query string there. If the original URL doesn't have any query string, we should add the ?.

The original commit came from @amazonwebservices and @trevorrowe in #579.
I had to revise the commit and make sure all the test cases are passing.
All credits still goes to those guys, thanks a lot!

    This can be used in migrations, i.e.

      create_table :users do |t|
        t.has_attached_file :avatar
      end

(three commits squashed into one by Alexey Mahotkin <squadette@gmail.com>)

Separate the UrlGenerator out from the Attachment. Some example plugins that could be written include generating thumbnails on the fly for different thumbnail sizes, or delaying the thumbnail generation until it is first called.

Fix a bug when passing a method name to `:if` and `:unless` option in `validates_attachment_presence`

Fixes #631