[rubygems/rubygems] Better way to join path components

The current way works, but error messages show duplicate "/" in paths,
which is weird.

https://github.com/rubygems/rubygems/commit/9123deb4fa

spec/bundler/support/artifice/vcr.rb

@@ -69,7 +69,7 @@ class BundlerVCRHTTP < Net::HTTP
     end
 
     def file_name_for_key(key)
-      key.join("/").gsub(/[\:*?"<>|]/, "-")
+      File.join(*key).gsub(/[\:*?"<>|]/, "-")
     end
 
     def request_pair_paths