This adds functionality to add additional params to the querystring for
S3 expiring URLs. The reason for this is if you want to override
response_content_type or response_content_disposition with expiring
URLs, you have to change Amazon's signature, which gets signed before
adding additional options to the querystring. Because it's added later,
and because Amazon's signature includes the full URL, the signature is
bad and the request fails.

To use this feature:

    has_attached_file :avatar,
                      :s3_url_options => { :response_content_disposition => "inline" }

Additionally, you can pass a lambda and it'll be evaluated when the URL
is generated. If, for example, the content type is incorrect in Amazon
(either empty or application/octet-stream), you can effectively cast the
response from Amazon as a particular content type.

    has_attached_file :avatar,
                      :s3_url_options => lambda {|model| { :response_content_type => model.avatar_content_type } }