add debug logging

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 12791ca..157c5fd 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -1945,6 +1945,9 @@
         }
     }
 
+    ngx_log_debug1(NGX_LOG_DEBUG_HTTP, c->log, 0,
+                   "http cacheable: %d", u->cacheable);
+
 #endif
 
     p = u->pipe;