Upstream: keepalive flag.

This patch introduces r->upstream->keepalive flag, which is set by protocol
handlers if connection to upstream is in good state and can be kept alive.

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 62009f6..4230abc 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1297,6 +1297,8 @@
         return NGX_ERROR;
     }
 
+    u->keepalive = 0;
+
     ngx_memzero(&u->headers_in, sizeof(ngx_http_upstream_headers_in_t));
     u->headers_in.content_length_n = -1;
 
@@ -2006,6 +2008,11 @@
         }
     }
 
+    if (u->length == 0) {
+        ngx_http_upstream_finalize_request(r, u, 0);
+        return;
+    }
+
     if (ngx_handle_read_event(rev, 0) != NGX_OK) {
         ngx_http_upstream_finalize_request(r, u, NGX_ERROR);
         return;
@@ -2126,7 +2133,7 @@
                 return;
             }
 
-            if (u->peer.connection->read->ready) {
+            if (u->peer.connection->read->ready || u->length == 0) {
                 ngx_http_upstream_process_non_buffered_upstream(r, u);
             }
         }

src/http/ngx_http_upstream.h

diff --git a/src/http/ngx_http_upstream.h b/src/http/ngx_http_upstream.h
index c3f706e..6e62044 100644
--- a/src/http/ngx_http_upstream.h
+++ b/src/http/ngx_http_upstream.h
@@ -308,6 +308,7 @@
 #endif
 
     unsigned                         buffering:1;
+    unsigned                         keepalive:1;
 
     unsigned                         request_sent:1;
     unsigned                         header_sent:1;