Added date to header

In message header, `date` field is required.

Ref: https://jupyter-client.readthedocs.io/en/latest/messaging.html#message-header

Author: ebababi

lib/iruby/session.rb

@@ -2,6 +2,7 @@
 require 'iruby/session/mixin'
 
 require 'securerandom'
+require 'time'
 
 module IRuby
   class Session
@@ -79,6 +80,7 @@ def send(socket_type, message_type, content)
       header = {
         msg_type: message_type,
         msg_id:   SecureRandom.uuid,
+        date:     Time.now.utc.iso8601,
         username: 'kernel',
         session:  @session_id,
         version:  '5.0'