BUG/MINOR: cache/htx: Return only the headers of cached objects to HEAD requests

BUG/MINOR: cache/htx: Return only the headers of cached objects to HEAD requests

The body of a cached object must not be sent in response to a HEAD request. This
works for the legacy HTTP because the parsing is performed by HTTP analyzers
_AND_ because the connection is closed at the end of the transaction. So the
body is ignored. But the applet send it. For the HTX, the applet must skip the
body explicitly.

This patch must be backported to 1.9.