Halve memory allocation in S3Service#download

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

activestorage/lib/active_storage/service/s3_service.rb

@@ -33,7 +33,7 @@ module ActiveStorage
         end
       else
         instrument :download, key: key do
-          object_for(key).get.body.read.force_encoding(Encoding::BINARY)
+          object_for(key).get.body.string.force_encoding(Encoding::BINARY)
         end
       end
     end