also affects pipe and pass processing.

Move the HEAD->GET override to vcl_miss{} and do it on the bereq.* so
we do not tamper with the original request.

Fixes:	#679



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4662 d4fa192b-c00b-0410-8231-f00ffab90ce4