* lib/rubygems/resolver.rb (sort_dependencies): use stable sort.
TestGemRequestSetLockfile#test_to_s_gem_dependency_non_default
fails because this method return unstable results.
Note that Enumerable#sort_by is unstable.
I'm not sure the "stable" nature is required for RubyGems.
The fact is that using stable sort, the test passed on
mswin64+VS2017 where the sort results was reverse (unstable) order.
Also using `-i` instead of `i` (it means forcing unstable sort)
this test fails on other platform where the test successed before.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59502 b2dd03c8-39d4-4d8f-98ff-823fe69b080e