remove unnecessary unshift

* ext/pathname/lib/pathname.rb (Pathname#join): remove unnecessary unshift.
* test/pathname/test_pathname.rb (TestPathname#test_join): add tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45407 b2dd03c8-39d4-4d8f-98ff-823fe69b080e