restore server[:DocumentRootOptions] setting.

Surprisingly (at least for me), `server[:DocumentRootOptions]`
on Webrick is global information and it affect the result of
test_short_filename. Random order test fails because of global
information change. I doubt it is bug, but to fix random order
test, I restore the value.

test/webrick/test_filehandler.rb

@@ -177,7 +177,8 @@ def test_filehandler
         assert_equal("206", res.code, log.call)
         assert_equal("multipart/byteranges", res.content_type, log.call)
       }
-
+    ensure
+      server[:DocumentRootOptions].delete :NondisclosureName
     end
   end