Minor optimization to set buffer size appropriately

This avoids unnecessary allocations/reallocations while decoding a message by pre-sizing the buffer to the amount we know it will require. I feel like there's probably a better way to handle this, but this works for now.

messages/SLMessage.js

@@ -3,8 +3,14 @@
 const SmartBuffer = require('smart-buffer').SmartBuffer;
 
 exports.SLMessage = class SLMessage extends SmartBuffer {
-  constructor(senderId, messageId) {
-    super();
+  constructor(senderId, messageId, size) {
+    var options;
+    if (size) {
+      options = {
+        size: size,
+      };
+    }
+    super(options);
 
     if (typeof senderId === 'number' || typeof senderId === 'undefined') {
       this.writeUInt16LE(senderId || 0);