* lib/rubygems/config_file.rb: Lazily load .gem/credentials to only
check permissions when necessary. RubyGems bug #465
* test/rubygems/test_gem_config_file.rb: Test for the above.
* test/rubygems/test_gem_commands_push_command.rb: Remove duplicated
test.
merge revision(s) 39491: [Backport #8403]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40731 b2dd03c8-39d4-4d8f-98ff-823fe69b080e