Revert r58345 and r58371.
These changes break the behavior of default gems. Bug #13428 says
r58345 is reasonable because gemspec file is installed by `to_ruby_for_cache`
method. But I revert `to_ruby_for_cache` in rbinstall.rb at r58403.
There is no reason that we apply r58345 now.
But I'm not sure about gemspec of default gems affects standalone gems.
I'm going to investigate it on rubygems/rubygems.
[Bug #15500][ruby-core:90867]
merge revision(s) 66867: [Backport #15500]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@66913 b2dd03c8-39d4-4d8f-98ff-823fe69b080e