test_rdoc_generator_json_index.rb: leaked fd

* test/rdoc/test_rdoc_generator_json_index.rb (test_generate_gzipped):
  fix leaked file descriptor.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48979 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/rdoc/test_rdoc_generator_json_index.rb

@@ -148,8 +148,9 @@ class TestRDocGeneratorJsonIndex < RDoc::TestCase
     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 = /