[rubygems/rubygems] Use the dedicated method to convert file path

The dedicated method `File.path` to deal with pathname-like objects
has been provided since ruby 1.9.0.
Also adds a test for rubygems/rubygems#6837.

https://github.com/rubygems/rubygems/commit/258c6eda80

lib/bundler/rubygems_integration.rb

@@ -245,7 +245,8 @@ module Bundler
       [::Kernel.singleton_class, ::Kernel].each do |kernel_class|
         kernel_class.send(:alias_method, :no_warning_require, :require)
         kernel_class.send(:define_method, :require) do |file|
-          name = file.to_s.tr("/", "-")
+          file = File.path(file)
+          name = file.tr("/", "-")
           if (::Gem::BUNDLED_GEMS::SINCE.keys - specs.to_a.map(&:name)).include?(name)
             unless $LOADED_FEATURES.any? {|f| f.end_with?("#{name}.rb", "#{name}.#{RbConfig::CONFIG["DLEXT"]}") }
               target_file = begin

lib/rubygems/core_ext/kernel_require.rb

@@ -37,8 +37,7 @@ module Kernel
     return gem_original_require(path) unless Gem.discover_gems_on_require
 
     RUBYGEMS_ACTIVATION_MONITOR.synchronize do
-      path = path.to_path if path.respond_to? :to_path
-      path = String.try_convert(path) || path
+      path = File.path(path)
 
       if spec = Gem.find_unresolved_default_spec(path)
         # Ensure -I beats a default gem

spec/bundler/runtime/setup_spec.rb

@@ -1650,4 +1650,34 @@ end
 
     expect(err).to include("net-imap is not part of the default gems")
   end
+
+  it "calls #to_path on the name to require" do
+    build_repo4 do
+      build_gem "net-imap" do |s|
+        s.write "lib/net/imap.rb", "NET_IMAP = '0.0.1'"
+      end
+      build_gem "csv"
+    end
+
+    install_gemfile <<-G
+      source "#{file_uri_for(gem_repo4)}"
+      gem "csv"
+    G
+
+    ruby <<-R
+      Gem.send(:remove_const, :BUNDLED_GEMS) if defined?(Gem::BUNDLED_GEMS)
+      module Gem::BUNDLED_GEMS
+        SINCE = { "csv" => "1.0.0", "net-imap" => "0.0.1" }
+      end
+      path = BasicObject.new
+      def path.to_path; 'net/imap'; end
+      require 'bundler/setup'
+      begin
+        require path
+      rescue LoadError
+      end
+    R
+
+    expect(err).to include("net-imap is not part of the default gems")
+  end
 end