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.

src/http/modules/ngx_http_stub_status_module.c

@@ -103,16 +103,6 @@ ngx_http_stub_status_handler(ngx_http_request_t *r)
     ngx_str_set(&r->headers_out.content_type, "text/plain");
     r->headers_out.content_type_lowcase = NULL;
 
-    if (r->method == NGX_HTTP_HEAD) {
-        r->headers_out.status = NGX_HTTP_OK;
-
-        rc = ngx_http_send_header(r);
-
-        if (rc == NGX_ERROR || rc > NGX_OK || r->header_only) {
-            return rc;
-        }
-    }
-
     size = sizeof("Active connections:  \n") + NGX_ATOMIC_T_LEN
            + sizeof("server accepts handled requests\n") - 1
            + 6 + 3 * NGX_ATOMIC_T_LEN

src/http/ngx_http_core_module.c

@@ -1782,7 +1782,7 @@ ngx_http_send_response(ngx_http_request_t *r, ngx_uint_t status,
         }
     }
 
-    if (r->method == NGX_HTTP_HEAD || (r != r->main && val.len == 0)) {
+    if (r != r->main && val.len == 0) {
         return ngx_http_send_header(r);
     }