fix headers size calculation

c/response.c

@@ -97,7 +97,7 @@ static size_t calc_res_headers_sz(const rpd_keyval *headers)
 
     while (i < headers->size) {
         size += strlen(headers->items[i].key);
-        size++; /* plus space */
+        size += 2; /* plus ": " */
         size += strlen(headers->items[i].val);
         size += 2; /* plus CRLF */
         i++;