Fix reaction count on new messages

[enebe-nb]

The message object returned by the discord api don't always have a reactions property. This must be created before to cache the message, or it will use the array object instanced on lib/models/Message.js. In the current behaviour all messages cached while not having a reaction, will have the reference for the same Array Object.

[qeled]

Would be better to fix it this way since sharing an empty array for messages with no reactions actually saves memory:

   if (e.type === Events.MESSAGE_REACTION_ADD) {
-    if (!message.reactions) {
+    if (!message.reactions || !message.reactions.length) {
       message = message.merge({reactions: []});

I can rebase the PR and edit it on my end, will this be fine?

[enebe-nb]

This way the whole Message object will be replicated every time it gets the first reaction (in case someone remove the reaction). It will work, so there's no problem.

[qeled]

Rebased.