[core] fix $REQUEST_HEADER[...] parsing in config
x-ref:
"Allow matching against any arbitrary HTTP header in the configuration file regexps"
@@ -1024,7 +1024,7 @@ static int config_tokenizer(server *srv, tokenizer_t *t, int *token_id, buffer *
if (t->in_cond) {
for (i = 0; t->input[t->offset + i] &&
(isalpha((unsigned char)t->input[t->offset + i])
- ); i++);
+ || t->input[t->offset + i] == '_'); ++i);
if (i && t->input[t->offset + i]) {
tid = TK_SRVVARNAME;
