MINOR: mux-h2: Handle RAW_DATA blocks when sending data

MINOR: mux-h2: Handle RAW_DATA blocks when sending data

Tunneled data are now stored in RAW_DATA blocks. The H2 multiplexer must be
able to send them, to be able to forward a tunnel established on the other
side.

For the H2 multiplexer, there is no difference with HTTP data: both are sent
in DATA frames. Only the ability to handle the new type is added here. No
producer emits such block yet.