- Bug report #1792649 (http://curl.haxx.se/bug/view.cgi?id=1792649) pointed
  out a problem with doing an empty upload over FTP on a re-used connection.
  I added test case 541 to reproduce it and to verify the fix.

- I noticed while writing test 541 that the FTP code wrongly did a CWD on the
  second transfer as it didn't store and remember the "" path from the
  previous transfer so it would instead CWD to the entry path as stored. This
  worked, but did a superfluous command. Thus, test case 541 now also verifies
  this fix.