Added r->limit_rate_after.

As of now, it allows to better control bandwidth limiting from additional
modules.  It is also expected to be used to add variables support to
the limit_rate_after directive.

src/http/ngx_http_request.h

diff --git a/src/http/ngx_http_request.h b/src/http/ngx_http_request.h
index bd842df..1babeb2 100644
--- a/src/http/ngx_http_request.h
+++ b/src/http/ngx_http_request.h
@@ -420,6 +420,7 @@
 #endif
 
     size_t                            limit_rate;
+    size_t                            limit_rate_after;
 
     /* used to learn the Apache compatible response length without a header */
     size_t                            header_size;

src/http/ngx_http_write_filter_module.c

diff --git a/src/http/ngx_http_write_filter_module.c b/src/http/ngx_http_write_filter_module.c
index 5594c7f..f74dbc8 100644
--- a/src/http/ngx_http_write_filter_module.c
+++ b/src/http/ngx_http_write_filter_module.c
@@ -207,8 +207,12 @@
     }
 
     if (r->limit_rate) {
+        if (r->limit_rate_after == 0) {
+            r->limit_rate_after = clcf->limit_rate_after;
+        }
+
         limit = (off_t) r->limit_rate * (ngx_time() - r->start_sec + 1)
-                - (c->sent - clcf->limit_rate_after);
+                - (c->sent - r->limit_rate_after);
 
         if (limit <= 0) {
             c->write->delayed = 1;
@@ -249,14 +253,14 @@
 
         nsent = c->sent;
 
-        if (clcf->limit_rate_after) {
+        if (r->limit_rate_after) {
 
-            sent -= clcf->limit_rate_after;
+            sent -= r->limit_rate_after;
             if (sent < 0) {
                 sent = 0;
             }
 
-            nsent -= clcf->limit_rate_after;
+            nsent -= r->limit_rate_after;
             if (nsent < 0) {
                 nsent = 0;
             }