bug - local tx submission is not backward compatible

[ashisherc]

please see IntersectMBO/ouroboros-consensus#137

Summary
I am using the local transaction submission mini protocol, while submitting a transaction that is built using Alonzo spec, and being submitted with era number 5 (Babbage), the node disconnects the connection and does not return an error message as well.

Steps to reproduce
Build an Alonzo era tx,
Prepare payload with era as 5
Submit using the local transaction submission mini protocol

Expected behaviour
Since Babbage era tx spec is Alonzo compatible, a tx built using Alonzo only spec is a valid Babbage era tx. The protocol and node should accept the tx.