Forces read options to be a hash (regression)

With 0.4.0 unspecified read options are send as nil to the aws-sdk, which causes a `NoMethodError: undefined method `[]=' for nil:NilClass` error in the aws-sdk.

```Stacktrace (most recent call first):

  aws/s3/s3_object.rb:1077:in `read'
    options[:bucket_name] = bucket.name
  carrierwave/storage/aws.rb:68:in `read'
    file.read(uploader_read_options)```

The casting was removed during this refactoring: https://github.com/sorentwo/carrierwave-aws/commit/50608f019649f9c7582c8de51f68c0b71c308cd7