Fix send-packets and receive-packets encoding

When logging received bgp messages, message type is already converted
to integer in Connection.reader (exabgp/lib/exabgp/reactor/network/connection.py:226).
This causes exceptions, when receive-packets is enabled for watcher
process. Fix is to change encoding api to expect message type as an
integer.
Tested by running exabgp with send-packets&receive-packets watcher in
both text and json encodings.