Don't use fast tls on mingw and cygwin

Summary: Mingw and cygwin both use a pthreads layer for it's __thread usage instead of native tls

Reviewed By: @JoelMarcey

Differential Revision: D1500175
Fix issues with empty, chunked response (retry)

Summary: We call sendRaw at the end of a request, which normally causes
headers to be sent (if they haven't already been sent), and does other
"response started" things. If the response is empty, and chunked encoding
has been requested, we skip sending it.

The result is that onSendEnd fails to close the request, and the client
is left dangling until it times out.

Always send, even if the payload is empty.

Reviewed By: @jasone

Differential Revision: D1499733