fix sending a cached file using AIO

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 11a2cc6..6dcebd0 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -439,11 +439,13 @@
 
         rc = ngx_http_upstream_cache(r, u);
 
-        if (rc == NGX_AGAIN) {
+        if (rc == NGX_BUSY) {
             r->write_event_handler = ngx_http_upstream_init_request;
             return;
         }
 
+        r->write_event_handler = ngx_http_request_empty_handler;
+
         if (rc == NGX_DONE) {
             return;
         }
@@ -704,7 +706,7 @@
 
     case NGX_AGAIN:
 
-        return NGX_AGAIN;
+        return NGX_BUSY;
 
     case NGX_ERROR: