MINOR: quic: Prepare STREAM frames to fill QUIC packets

MINOR: quic: Prepare STREAM frames to fill QUIC packets

We must take as most as possible data from STREAM frames to be encapsulated
in QUIC packets, almost as this is done for CRYPTO frames whose fields are
variable length fields. The difference is that STREAM frames are only accepted
for short packets without any "Length" field. So it is sufficient to call
max_available_room() for that in place of max_stream_data_size() as this
is done for CRYPTO data.