HEAD: don't force-close after response-headers

A HEAD response has no body length and gets the headers like the
corresponding GET would so it should not get closed after the response
based on the same rules. This mistake caused connections that did HEAD
to get closed too often without a valid reason.

Bug: http://curl.haxx.se/bug/view.cgi?id=3542731
Reported by: Eelco Dolstra