backport r48979 from ruby/ruby

Author: hsbt

test/test_rdoc_generator_json_index.rb

@@ -152,8 +152,9 @@ def test_generate_gzipped
     assert_file 'js/search_index.js'
     assert_file 'js/search_index.js.gz'
 
-    gzip = File.open 'js/search_index.js.gz'
-    json = Zlib::GzipReader.new(gzip).read
+    json = File.open('js/search_index.js.gz') {|gzip|
+      Zlib::GzipReader.new(gzip).read
+    }
 
     json =~ /\Avar search_data = /