[rubygems/rubygems] Test platform parsing stability

On past versions there were observed cases of inconsistencies when some
platforms were re-parsed.

Ensure that a platform's string representation parses again in a
platform object equal to the original.

https://github.com/rubygems/rubygems/commit/6da35ee93c

test/rubygems/test_gem_platform.rb

@@ -144,6 +144,7 @@ class TestGemPlatform < Gem::TestCase
     test_cases.each do |arch, expected|
       platform = Gem::Platform.new arch
       assert_equal expected, platform.to_a, arch.inspect
+      assert_equal expected, Gem::Platform.new(platform.to_s).to_a, arch.inspect
     end
   end