Removed incorrect optimization of HEAD requests.

Removed incorrect optimization of HEAD requests.

The stub status module and ngx_http_send_response() (used by the empty gif
module and the "return" directive) incorrectly assumed that responding
to HEAD requests always results in r->header_only being set.  This is not
true, and results in incorrect behaviour, for example, in the following
configuration:

   location / {
       image_filter size;
       return 200 test;
   }

Fix is to remove this incorrect micro-optimization from both stub status
module and ngx_http_send_response().

Reported by Chris Newton.