more POST-after-PUT confusion

When doing HTTP(S) transfers, libcurl might erroneously use the read callback
(CURLOPT_READFUNCTION) to ask for data to send, even when the
CURLOPT_POSTFIELDS option has been set, if the same handle previously was
used to issue a PUT request which used that callback.

This flaw may surprise the application and cause it to misbehave and either
send off the wrong data or use memory after free or similar in the second
transfer.

The problem exists in the logic for a reused handle when it is (expected to
be) changed from a PUT to a POST.