Only update the message encoding

lib/logstash-logger/formatter/base.rb

@@ -70,11 +70,9 @@ def format_event(event)
       end
 
       def force_utf8_encoding(event)
-        new_event = event.dup
-        new_data = {}
-        new_event.instance_variable_get(:@data).each { |k, v| new_data[k] = v.to_s.force_encoding('UTF-8') }
-        new_event.instance_variable_set(:@data, new_data)
-        new_event
+        original_message = event.instance_variable_get(:@data)['message']
+        event.message = original_message.force_encoding('UTF-8')
+        event
       end
     end
   end

spec/formatter/base_spec.rb

@@ -28,15 +28,14 @@
   describe '#force_utf8_encoding' do
     let(:event) { LogStash::Event.new("message" => "foo".force_encoding('ASCII-8BIT')) }
 
-    it 'returns a new event' do
-      expect(subject.send(:force_utf8_encoding, event)).not_to eq(event)
+    it 'returns the same event' do
+      expect(subject.send(:force_utf8_encoding, event)).to eq(event)
     end
 
-    it 'forces all event data to UTF-8 encoding' do
-      updated_event_data = subject.send(:force_utf8_encoding, event).instance_variable_get(:@data)
-      updated_event_data.each_value do |value|
-        expect(value.encoding.name).to eq('UTF-8')
-      end
+    it 'forces the event message to UTF-8 encoding' do
+      subject.send(:force_utf8_encoding, event)
+      updated_event_data = event.instance_variable_get(:@data)
+      expect(updated_event_data['message'].encoding.name).to eq('UTF-8')
     end
   end