cache GET requests only

src/http/ngx_http_upstream.c

diff --git a/src/http/ngx_http_upstream.c b/src/http/ngx_http_upstream.c
index 1ff6ae4..76f1bc3 100644
--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -531,6 +531,10 @@
     ngx_int_t          rc;
     ngx_http_cache_t  *c;
 
+    if (!(r->method & NGX_HTTP_GET)) {
+        return NGX_DECLINED;
+    }
+
     c = ngx_pcalloc(r->pool, sizeof(ngx_http_cache_t));
     if (c == NULL) {
         return NGX_ERROR;