pathname.rb: UNC root pathname needs a separator
* ext/pathname/lib/pathname.rb (Pathname#plus): UNC root pathname
needs a separator. File.basename returns "/" on UNC root, as
well as sole drive letter, even if it does not end with a
separator. [ruby-core:80900] [Bug #13515]
merge revision(s) 59102: [Backport #13515]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59400 b2dd03c8-39d4-4d8f-98ff-823fe69b080e