nginx: fix missing headers while SecResponseBodyAccess was On

The problem was caused by the fact that ModSecutiry was telling Nginx that
headers had been sent when, in fact, had not. This modification was added in
the past, along with others, in the following commit: fd2c30fa23
This patch, just removes the "r->header_sent = 1". After that modification
the following regression tests started to work again:

from: action/00-disruptive-actions.t
   3) action - pass in phase:3
   4) action - pass in phase:4
  11) action - deny in phase:3
  12) action - deny in phase:4
  19) action - redirect in phase:3 (get)
  20) action - redirect in phase:4 (get)
  23) action - proxy in phase:3 (get)
  24) action - proxy in phase:4 (get)
from: config/10-response-directives.t
   2) config - SecResponseBodyAccess On
   6) config - SecResponseBodyLimit (greater)
   7) config - SecResponseBodyLimitAction Reject

nginx/modsecurity/ngx_http_modsecurity.c

@@ -1139,9 +1139,6 @@ ngx_http_modsecurity_header_filter(ngx_http_request_t *r) {
 
     /* SecResponseBodyAccess on, process rules in body filter */
 
-    /* pretend we are ngx_http_header_filter */
-    r->header_sent = 1;
-
     r->filter_need_in_memory = 1;
     return NGX_OK;
 }