Already-scheduled events might not have correlation_id

Scenario:
- an event is published but not yet picked by background queue
- RES version is bumped and is expecting correlation_id presence in
  CorrelatedHandler
- queued event arrives to handler

Not immediately obvious scenario, test added so that mutant is aware of
expectations.

Even more common when replaying an event (retries).

rails_event_store/lib/rails_event_store/async_handler_helpers.rb

@@ -10,8 +10,8 @@ def perform(payload)
   module CorrelatedHandler
     def perform(event)
       Rails.configuration.event_store.with_metadata(
-        correlation_id: event.metadata.fetch(:correlation_id),
-        causation_id: event.event_id
+        correlation_id: event.metadata[:correlation_id],
+        causation_id:   event.event_id
       ) do
         super
       end

rails_event_store/spec/rails_event_store_spec.rb

@@ -136,8 +136,29 @@ def verify(subscriber)
     expect(SidekiqHandlerWithHelper.event).to eq(ev)
   end
 
+  specify 'CorrelatedHandler with event not yet scheduled with correlation_id' do
+    MetadataHandler.metadata = nil
+    event_store.append(ev = RailsEventStore::Event.new)
+    ev = event_store.read.event(ev.event_id)
+    MetadataHandler.perform_now(serialize_without_correlation_id(ev))
+    expect(MetadataHandler.metadata).to eq({
+      correlation_id: nil,
+      causation_id:   ev.event_id,
+    })
+  end
+
   private
 
+  def serialize_without_correlation_id(ev)
+    serialized =
+      event_store
+        .__send__(:mapper)
+        .event_to_serialized_record(ev)
+        .to_h
+    serialized[:metadata] = "---\n:timestamp: 2020-11-02 14:32:53.747308000 Z\n"
+    serialized
+  end
+
   def wait_until(&block)
     Timeout.timeout(1) do
       loop do