why isn't this AWS rails function url_for setting content-disposition for my request?
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i'm using rails 2.3.14 with ruby 1.8.7.  i'm generating authenticated URLs to get S3 objects, and i'm using the latest aws-sdk gem.  i've seen this answer and my code is the same : http://stackoverflow.com/questions/10429050/aws-s3-ruby-url-for-response-headers-custom-file-name-and-file-type

here's my code.  when i hit the url that this generates, it does not set the content-disposition header in the response, it seems to ignore that option completely.  what am i doing wrong?

    AWS::S3::S3Object.url_for(key, bucket, 
        :authenticated => true, 
        :use_ssl => (ENV['RAILS_ENV'] == 'production'),
        :expires_in => S3_EXPIRE_TIME,
        :response_content_disposition => 'attachment;'   # seconds
        )