If HTTPAUTH_GSSNEGOTIATE was used for a POST request and
gss_init_sec_context() failed, the POST request was sent
with empty body. This commit also restores the original
behavior of `curl --fail --negotiate`, which was changed
by commit 6c60355323.
Add regression tests 2077 and 2078 to cover this.
Fixes#3992Closes#4171
