Support disabled mode

gems/aws-sdk-s3/lib/aws-sdk-s3/customizations/object.rb

@@ -498,7 +498,7 @@ def upload_file(source, options = {})
       #   checksum validation fails. You may provide a `on_checksum_validated`
       #   callback if you need to verify that validation occurred and which
       #   algorithm was used.  To disable checksum validation, set
-      #   `checksum_mode` to either `false` or `nil`.
+      #   `checksum_mode` to "DISABLED" or false.
       #
       # @option options [Callable] on_checksum_validated Called each time a
       #   request's checksum is validated with the checksum algorithm and the

gems/aws-sdk-s3/lib/aws-sdk-s3/file_downloader.rb

@@ -33,10 +33,10 @@ def download(destination, options = {})
         @params[:version_id] = options[:version_id] if options[:version_id]
 
         # checksum_mode only supports the value "ENABLED"
-        # falsey values (false/nil) should be considered disabled and
-        # the api parameter should be unset.
-        if options.fetch(:checksum_mode, 'ENABLED')
-          @params[:checksum_mode] = options.fetch(:checksum_mode, 'ENABLED')
+        # falsey values (false/nil) or "DISABLED" should be considered
+        # disabled and the api parameter should be unset.
+        if (checksum_mode = options.fetch(:checksum_mode, 'ENABLED'))
+          @params[:checksum_mode] = checksum_mode unless checksum_mode.upcase == 'DISABLED'
         end
         @on_checksum_validated = options[:on_checksum_validated]
 

gems/aws-sdk-s3/spec/object/download_file_spec.rb

@@ -231,7 +231,7 @@ module S3
             response_target: path
           }).exactly(1).times
 
-          small_obj.download_file(path, checksum_mode: nil)
+          small_obj.download_file(path, checksum_mode: 'DISABLED')
         end
 
         it 'raises an error if an invalid mode is specified' do