fix gem_prelude.rb for Ruby 1.9.1

--- ruby-1.9.1-p431/gem_prelude.rb	(revision 25931)
+++ ruby-1.9.1-p431-fixed/gem_prelude.rb	(revision 25932)
@@ -66,8 +66,9 @@
     end
 
     def self.set_home(home)
-      @gem_home = home.force_encoding(Encoding.find('filesystem'))
-      ensure_gem_subdirectories(@gem_home)
+      home = home.dup.force_encoding(Encoding.find('filesystem'))
+      home.gsub!(File::ALT_SEPARATOR, File::SEPARATOR) if File::ALT_SEPARATOR
+      @gem_home = home
     end
 
     def self.set_paths(gpaths)