merge revision(s) 39491: [Backport #8403]

	* 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.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@40731 b2dd03c8-39d4-4d8f-98ff-823fe69b080e