[ruby/rubygems] Use Pathname#absolute?

`Pathname::SEPARATOR_PAT` should be private, but was not set to
private just due to a typo.

ruby/rubygems@67ce6df4c9

Author: nobu

lib/bundler/source/path.rb

@@ -220,10 +220,11 @@ def generate_bin(spec, options = {})
         # Some gem authors put absolute paths in their gemspec
         # and we have to save them from themselves
         spec.files = spec.files.filter_map do |path|
-          next path unless /\A#{Pathname::SEPARATOR_PAT}/o.match?(path)
+          pathname = Pathname.new(path)
+          next path unless pathname.absolute?
           next if File.directory?(path)
           begin
-            Pathname.new(path).relative_path_from(gem_dir).to_s
+            pathname.relative_path_from(gem_dir).to_s
           rescue ArgumentError
             path
           end