Backport fix of test failure on Windows

test_generate_bin_bindir_with_user_install_warning(TestGemInstaller) fails
on Windows with msys bash. It makes comparing paths case-insensitive.

See:
#299 (comment)

lib/rubygems/installer.rb

@@ -435,7 +435,13 @@ def process_options
   def check_that_user_bin_dir_is_in_path
     user_bin_dir = @bin_dir || Gem.bindir(gem_home)
     user_bin_dir.gsub!(File::SEPARATOR, File::ALT_SEPARATOR) if File::ALT_SEPARATOR
-    unless ENV['PATH'].split(File::PATH_SEPARATOR).include? user_bin_dir then
+    path = ENV['PATH']
+    if Gem.win_platform? then
+      path = path.downcase
+      user_bin_dir = user_bin_dir.downcase
+    end
+
+    unless path.split(File::PATH_SEPARATOR).include? user_bin_dir then
       unless self.class.path_warning then
         alert_warning "You don't have #{user_bin_dir} in your PATH,\n\t  gem executables will not run."
         self.class.path_warning = true