Halve memory allocation in S3Service#download

[janko]

Aws::S3::Object#get returns a response with object content wrapped in an in-memory StringIO object. StringIO#read will return a copy of the content, which is not necessary because we can return the content directly using StringIO#string.

require "stringio"

content  = "foo"
stringio = StringIO.new(content)

content.object_id         # => 70113512953300
stringio.string.object_id # => 70113512953300
stringio.read.object_id   # => 70113512951780

This halves memory allocation of S3Service#download, because we remove the extra content duplication.

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