samples: sockets: dumb_http_server: Use big payload by default

This sends out 2KB+ payload (i.e. guaranteedly more than 1 network
packet). When this sample was initially written, using such payload
quickly let to a deadlock somewhere in the network stack. However
as of now, running with such payload can sustain testing with
"ab -n10000" (10000 consecutive HTTP requests using Apache Bench),
so set is as a default, to serve as a mark point against possible
future regressions.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>