[rubygems/rubygems] Fix checksum

Calculate the checksum of the content, not the given pathname at
the build time itself.

https://github.com/rubygems/rubygems/commit/b60ee97ee9

lib/bundler/gem_helper.rb

@@ -107,7 +107,7 @@ module Bundler
       SharedHelpers.filesystem_access(File.join(base, "checksums")) {|p| FileUtils.mkdir_p(p) }
       file_name = "#{File.basename(built_gem_path)}.sha512"
       require "digest/sha2"
-      checksum = ::Digest::SHA512.new.hexdigest(built_gem_path.to_s)
+      checksum = ::Digest::SHA512.file(built_gem_path).hexdigest
       target = File.join(base, "checksums", file_name)
       File.write(target, checksum)
       Bundler.ui.confirm "#{name} #{version} checksum written to checksums/#{file_name}."

spec/bundler/bundler/gem_helper_spec.rb

@@ -169,6 +169,12 @@ RSpec.describe Bundler::GemHelper do
     end
 
     describe "#build_checksum" do
+      it "calculates SHA512 of the content" do
+        subject.build_checksum(Pathname(IO::NULL))
+        sha_path = app_path.join("checksums", "#{File.basename(IO::NULL)}.sha512")
+        expect(File.read(sha_path)).to eql(Digest::SHA512.hexdigest(""))
+      end
+
       context "when build was successful" do
         it "creates .sha512 file" do
           mock_build_message app_name, app_version