Fixes #843

`aws-sdk` has been updated to support newer version of nokogiri, so we
should loosen our dependency.

* Force column reset in Rails >= 3.1
* Reuse another helper method

ContentTypeValidator now honors the `:allow_nil` and `:allow_blank` option.

This will ensure that the content type is a String.

Fixes #805

Make sure that we close opened files after we're done with them.

AttachmentAdapter now handles this for us.

Fix problem with incorrect content_type from `file` command

Fixed a typo in README.md.

Remove init.rb since plugins were deprecated and we don't support Rails 2.3 anymore

2.3 anymore

Fixes AWS::S3::Errors::RequestTimeout on Model#save

* Add failing test for case when no logger is specified
* add fix for failing test
* fix teardown of no logger test

In Rails, `ActionDispatch::Http::UploadedFile` has an accessor to access
a `Tempfile` object. However, `Rack::Test::UploadedFile` does not
provide that, but provinding the `#path` method instead. To be able to
support both, we have to check for existance for `#tempfile` method,
then fallback to `#path` method.

Fixes #807