Kernel.exec will not use a modified ENV["PATH"] to locate commands on Windows

Demonstrate with this simple test:

• Create batch file: C:\testdir\batchdir\test.bat containing:

echo "This is test.bat"

• Create ruby file: C:\testdir\test.rb containing:

      paths = (ENV["PATH"] || "").split(File::PATH_SEPARATOR)
      paths.unshift "c:\\testdir\\batchdir"
      ENV["PATH"] = paths.uniq.join(File::PATH_SEPARATOR)
  
      Kernel.exec "test.bat"
  

• And run:

      C:\testdir>jruby test.rb
      test.rb:9: No such file or directory - test.bat (Errno::ENOENT)
  

• But using binary Ruby 1.8.6 (2008-08-11 patchlevel 287) [i386-mswin32] it works just fine:

      C:\testdir>ruby test.rb
  
      C:\testdir>echo "This is test.bat"
      "This is test.bat"
  

I found this when trying to use Bundler's 'bundle exec' command, which uses the same technique.