HTTP parser: restricting allowed characters in fields values.

According to RFC 7230 only printable 7-bit ASCII characters are allowed
in field values.

src/nxt_http_parse.c

@@ -679,7 +679,8 @@ nxt_http_lookup_field_end(u_char *p, u_char *end)
 
 #define nxt_field_end_test_char(ch)                                           \
                                                                               \
-        if (nxt_slow_path((ch) < 0x10)) {                                     \
+        /* Values below 0x20 become more than 0xdf. */                        \
+        if (nxt_slow_path((u_char) ((ch) - 0x20) > 0x5e)) {                   \
             return &(ch);                                                     \
         }