Incomplete implementation in SanitizedFile#read can lead to infinite loop

[mshibuya]

To address the point reported by #1959, CarrierWave::SanitizedFile#read was changed to accept optional arguments in 9096459. It didn't come with full features that are expected for IO#read, to cope with the existing implementation that normally reads all the content at once.

But it turned out that the incompleteness is very harmful. When combined with a legitimate implementation of chunked reading like this one, it results in infinite loop because SanitizedFile#read keeps returning the same content endlessly.
https://github.com/mongoid/mongoid-grid_fs/blob/878393f1ba990909c4519641926d3f8d77230407/lib/mongoid/grid_fs.rb#L465-L467

I found this when I was working on an attempt of making carrierwave-mongoid support carrierwave 3.x, and it was very hard to debug. We should have a proper fix for this.