Always add a @timestamp field in the logstash encoder, even if it was removed by a filter

lib/rackstash/encoders/logstash.rb

@@ -20,6 +20,8 @@ module Rackstash
       # @return [String] the event as a single-line JSON string
       def encode(event)
         event[FIELD_VERSION] = '1'.freeze if event[FIELD_VERSION].nil?
+        event[FIELD_TIMESTAMP] ||= Time.now.utc.freeze
+
         super(event)
       end
     end

spec/rackstash/encoders/logstash_spec.rb

@@ -14,7 +14,8 @@ describe Rackstash::Encoders::Logstash do
   describe '#encode' do
     it 'formats the passed event hash as a JSON string and includes @version' do
       event = { 'hello' => 'world', 'message' => ["hello\n", "world"] }
-      expect(encoder.encode(event)).to eql '{"hello":"world","message":"hello\nworld","@version":"1"}'
+      expect(encoder.encode(event))
+        .to match /\A\{"hello":"world","message":"hello\\nworld","@version":"1","@timestamp":"\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d\.\d{6}Z"\}\z/
     end
   end
 end