Merge RubyGems 2.7.7

see release details here: https://blog.rubygems.org/2018/05/18/2.7.7-released.html

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63461 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/test_utilities.rb

@@ -64,7 +64,7 @@ class Gem::FakeFetcher
       data.call
     else
       if path.to_s =~ /gz$/ and not data.nil? and not data.empty? then
-        data = Gem.gunzip data
+        data = Gem::Util.gunzip data
       end
 
       data