HTTP parser: relaxed checking of fields values.

Allowing characters up to 0xFF doesn't conflict with RFC 7230.
Particularly, this make it possible to pass unencoded UTF-8 data
through HTTP headers, which can be useful.

src/test/nxt_http_parse_test.c

@@ -301,7 +301,7 @@ static nxt_http_parse_test_case_t  nxt_http_test_cases[] = {
     {
         nxt_string("GET / HTTP/1.1\r\n"
                    "Host: пример.испытание\r\n\r\n"),
-        NXT_HTTP_PARSE_INVALID,
+        NXT_DONE,
         NULL, { NULL }
     },
     {