Move Session methods onto prototype so that it can be cloned

Functions cannot be cloned by the HTML5 structured clone algorithm. This
prevents the session being able to be written to an IndexedDb object store.

Author: joebandenburg

src/Session.js

@@ -25,38 +25,30 @@ import SessionState from "./SessionState";
  * @param {Session|Object} [session] - another session. If passed, this session will clone its state.
  * @constructor
  */
-function Session(session) {
-    var self = this;
-
-    var states = [];
-
-    if (session) {
-        for (let state of session.states) {
-            states.push(new SessionState(state));
+export default class Session {
+    constructor(session) {
+        this.states = [];
+        if (session) {
+            for (let state of session.states) {
+                this.states.push(new SessionState(state));
+            }
         }
+        Object.seal(this);
     }
 
-    Object.defineProperty(self, "states", {
-        get: () => states
-    });
-
-    self.mostRecentState = () => {
-        return states[0];
-    };
+    mostRecentState() {
+        return this.states[0];
+    }
 
-    self.addState = (state) => {
-        states.unshift(state);
-        if (states.length > ProtocolConstants.maximumSessionStatesPerIdentity) {
-            states.pop();
+    addState(state) {
+        this.states.unshift(state);
+        if (this.states.length > ProtocolConstants.maximumSessionStatesPerIdentity) {
+            this.states.pop();
         }
-    };
-
-    self.removeState = (state) => {
-        var index = states.indexOf(state);
-        states.splice(index, 1);
-    };
+    }
 
-    Object.freeze(this);
+    removeState(state) {
+        var index = this.states.indexOf(state);
+        this.states.splice(index, 1);
+    }
 }
-
-export default Session;