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