Index: lib/webrick/httpresponse.rb
===================================================================
--- lib/webrick/httpresponse.rb.orig	2008-06-06 10:05:24.000000000 +0200
+++ lib/webrick/httpresponse.rb	2010-07-01 17:58:35.585190988 +0200
@@ -209,7 +209,7 @@ module WEBrick
         @keep_alive = false
         self.status = HTTPStatus::RC_INTERNAL_SERVER_ERROR
       end
-      @header['content-type'] = "text/html"
+      @header['content-type'] = "text/html; charset=utf-8"
 
       if respond_to?(:create_error_page)
         create_error_page()