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

Author: spacewander

src/ngx_http_lua_headers_in.c

@@ -669,7 +669,13 @@ ngx_http_lua_set_input_header(ngx_http_request_t *r, ngx_str_t key,
         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;