Httpclient internally stores a copy of the certificate_check callback and
payload. When connecting via HTTPS, and if the server sends back
"Connection: close" after the first request, the following request would
attempt to re-use the httpclient and call the (now outdated) callback. In
particular for pygit2 this is a problem, since callbacks / payloads are only
valid for the duration of a libgit2 call, leading to a ffi.from_handle()
error and crashing the Python interpreter.
