Upstream keepalive: reset c->sent on cached connections.

The c->sent is reset to 0 on each request by server-side http code,
so do the same on client side.  This allows to count number of bytes
sent in a particular request.
diff --git a/src/http/modules/ngx_http_upstream_keepalive_module.c b/src/http/modules/ngx_http_upstream_keepalive_module.c
index c1c1c70..f738f0c 100644
--- a/src/http/modules/ngx_http_upstream_keepalive_module.c
+++ b/src/http/modules/ngx_http_upstream_keepalive_module.c
@@ -248,6 +248,7 @@
                            "get keepalive peer: using connection %p", c);
 
             c->idle = 0;
+            c->sent = 0;
             c->log = pc->log;
             c->read->log = pc->log;
             c->write->log = pc->log;