Fixes filtering tests of cmd/stun-nat-behaviour utility.
Previously it reused STUN message structure and updated it
with .Reset(). It causes message missing header and proper
request type. Consequently, such messages were not recognized
by stun servers and they did not respond. Lack of response
was always considered to be an "address and port dependent
filtering".

Instead of duplicating Message.Build() logic with
.WriteHeader() and other internals, I've just reused
common approach creating STUN message everywhere.