Fix Bundler.which not working on Windows

It was ignoring executable extensions, which is the only hint
`File.executable?` uses on Windows to figure out executability.

bundler/lib/bundler.rb

@@ -512,14 +512,18 @@ def mkdir_p(path, options = {})
     end
 
     def which(executable)
+      exts = (RbConfig::CONFIG["EXECUTABLE_EXTS"]&.split || []) | [RbConfig::CONFIG["EXEEXT"]]
+
       if File.file?(executable) && File.executable?(executable)
         executable
       elsif paths = ENV["PATH"]
         quote = '"'.freeze
         paths.split(File::PATH_SEPARATOR).find do |path|
           path = path[1..-2] if path.start_with?(quote) && path.end_with?(quote)
-          executable_path = File.expand_path(executable, path)
-          return executable_path if File.file?(executable_path) && File.executable?(executable_path)
+          exts.find do |ext|
+            executable_path = File.expand_path(executable + ext, path)
+            return executable_path if File.file?(executable_path) && File.executable?(executable_path)
+          end
         end
       end
     end

bundler/spec/bundler/bundler_spec.rb

@@ -159,7 +159,7 @@
     context "when the executable in inside a quoted path" do
       let(:expected) do
         if Gem.win_platform?
-          "C:/e/executable"
+          "C:/e/executable.cmd"
         else
           "/e/executable"
         end