BUG/MEDIUM: h2/hpack: fix emission of HPACK DTSU after settings change

BUG/MEDIUM: h2/hpack: fix emission of HPACK DTSU after settings change

Sadly, despite particular care, commit 39a0a1e12 ("MEDIUM: h2/hpack: emit
a Dynamic Table Size Update after settings change") broke H2 when sending
DTSU. A missing negation on the flag caused the DTSU_EMITTED flag to be
lost and the DTSU to be sent again on the next stream, and possibly to
break flow control or a few other internal states.

This will have to be backported wherever the patch above was backported.

Thanks to Yves Lafon for notifying us with elements to reproduce the
issue!