Fix automatic creation of bucket and set Content-Type & Content-Disposition

I haven't tried with previous releases of the AWS SDK, but with the current one (1.7.1) at least, there is no `create` method in the `Bucket` ([Doc](http://docs.amazonwebservices.com/AWSRubySDK/latest/AWS/S3/Bucket.html)) class. We need to use `BucketCollections.create` instead.