Make RateStore marshalable (add #marshal_dump)

[jtarchie]

Problem

Marshaling a Money::Bank::VariableExchange whose store is an
ActiveCurrency::RateStore fails, because RateStore has no #marshal_dump.

VariableExchange#marshal_load rebuilds the store with:

@store = store_info.shift.new(*store_info)

so it expects the store's #marshal_dump to return
[StoreClass, *constructor_args]. Without a custom #marshal_dump,
reloading a marshaled bank raises.

This comes up when the configured default_bank uses a RateStore and
something marshals it, e.g. a Rails app with a Marshal-based cache or
message serializer that caches the default bank.

Fix

Add #marshal_dump to RateStore returning [self.class]. RateStore
takes no constructor arguments, so that is all VariableExchange#marshal_load
needs to rebuild it on load.

Test

Added specs asserting RateStore#marshal_dump returns [described_class]
and that a VariableExchange backed by a RateStore round-trips through
Marshal.

[sunny]

Thank you! Could you pull the changes from main (updated GitHub test workflow) and add an entry to the CHANGELOG as well, please? 🙏🏻

[jtarchie]

Thanks for the quick review! Rebased onto the latest main (picking up the updated CI workflow) and added a CHANGELOG entry under Unreleased. Should be good to go 🙏