attempt to fix the notorious test 91 failures

                                  Changelog

Daniel(30 January 2004)

- Added code that attempts to fix the test 91 failure. As has been figured out

  by Patrick Smith, the error happens because we re-use a connection that the

  server is just about to close and we even manage to send away the request

  without seeing an error. On the first read attempt we get a ECONNRESET.

  Starting now, we attempt to detect this and if so, we retry the request on a

  fresh connection.

- I added test case 510 which is a custom program that does a POST using a

  read callback, with chunked transfer-encoding.