MINOR: http-htx: Report the payload of RAW_DATA blocks in internal.htx_blk.data

MINOR: http-htx: Report the payload of RAW_DATA blocks in internal.htx_blk.data

The "internal.htx_blk.data" sample fetch only reported the payload of DATA
blocks. Because tunneled data are now stored in RAW_DATA blocks, and because
"internal.htx_blk.type" reports this new type, an empty string was returned
for such block while its type was properly displayed.

These sample fetches are debugging helpers. So both types are now handled,
htx_is_data_type() being used to test it.