optimize: don't calculate hash when clearing the header.

Author: spacewander

src/ngx_http_lua_headers_out.c

@@ -499,7 +499,13 @@ ngx_http_lua_set_output_header(ngx_http_request_t *r, ngx_http_lua_ctx_t *ctx,
         return NGX_ERROR;
     }
 
-    hv.hash = ngx_hash_key_lc(key.data, key.len);
+    if (value.len > 0) {
+        hv.hash = ngx_hash_key_lc(key.data, key.len);
+
+    } else {
+        hv.hash = 0;
+    }
+
     hv.key = key;
 
     hv.offset = 0;