Fix custom message parsing in RTU framer

[wlcrs]

As the regression reported in #1695 still isn't resolved in pyModbus 3.4.1, I've been tinkering with the code to try to reproduce the issue.

I noticed that ModbusRtuFramer caches the function codes registered in the decoder lookup table. As the framer is already initialized before a client can call the register() function to register custom Modbus message types, this prevents those custom Modbus messages from being processed correctly.

This PR uses .keys() instead of set() which prevents function_codes from becoming outdated if a custom message class is registered.

Open question: if there a valid case for the decoder being None? Otherwise we can drop the fallback to {}?

[janiversen]

May I politely correct you, the issue raised in #1695 is solved ! seems you are having another issue in addition to #1695.

[janiversen]

LGTM.

[janiversen]

When you call register() it should update the local copy, but your solution is more robust.