Added write event handler reset in ngx_http_named_location().

On internal redirects this happens via ngx_http_handler() call, which is
not called on named location redirect.  As a result incorrect write handler
remained (if previously set) and this might cause incorrect behaviour (likely
request hang).

Patch by Yichun Zhang (agentzh).

src/http/ngx_http_core_module.c

@@ -2599,6 +2599,7 @@ ngx_http_named_location(ngx_http_request_t *r, ngx_str_t *name)
 
             r->phase_handler = cmcf->phase_engine.location_rewrite_index;
 
+            r->write_event_handler = ngx_http_core_run_phases;
             ngx_http_core_run_phases(r);
 
             return NGX_DONE;