Fixed throughput problems with large limit_rate.

Previous attempt to fix this was in r1658 (0.6.18), though that one wasn't
enough (it was a noop).
diff --git a/src/http/ngx_http_write_filter_module.c b/src/http/ngx_http_write_filter_module.c
index 4635134..f5dc6c1 100644
--- a/src/http/ngx_http_write_filter_module.c
+++ b/src/http/ngx_http_write_filter_module.c
@@ -262,7 +262,7 @@
             }
         }
 
-        delay = (ngx_msec_t) ((nsent - sent) * 1000 / r->limit_rate + 1);
+        delay = (ngx_msec_t) ((nsent - sent) * 1000 / r->limit_rate);
 
         if (delay > 0) {
             c->write->delayed = 1;