Restore the l_fin from Result.parent for injected frame.
+ cosmetic
removed io input request when rescueing do_send
Temporary correction in WS_FRAME.is_valid_utf_8_string_8 (..): BOOLEAN
(need to find better solution in term of performance)
Removed a few check assertion that should be handled as error (not assertions).
Avoid crashing when internal error in do_send
Do not  crash if on_close fails
Fixed UTF-8 validation for stream, and also string for the whole payload text message.
When error is met, return the invalid frame, and not the parent or fragment frame.
+ code cleaning
Fixing a few remaining issues with UTF-8 validation.
Cleaned and simplified utf-8 stream validation
Handling close frame inside a fragmented message