add SessioNEstablishedMessage and tests

ts/session/messages/outgoing/content/SessionEstablishedMessage.ts

@@ -8,6 +8,10 @@ export class SessionEstablishedMessage extends ContentMessage {
   }
 
   protected contentProto(): SignalService.Content {
-    throw new Error('Not implemented');
+    const nullMessage = new SignalService.NullMessage({});
+
+    return new SignalService.Content({
+      nullMessage,
+    });
   }
 }

ts/test/session/messages/SessionEstablishedMessage_test.ts

@@ -0,0 +1,25 @@
+import { expect } from 'chai';
+import { beforeEach} from 'mocha';
+
+import { SessionEstablishedMessage } from '../../../session/messages/outgoing';
+import { SignalService } from '../../../protobuf';
+
+describe('SessionEstablishedMessage', () => {
+    let message: SessionEstablishedMessage;
+    beforeEach(() => {
+        const timestamp = Date.now();
+        const identifier = '123456';
+        message = new SessionEstablishedMessage({timestamp, identifier});
+    });
+
+    it('has a nullMessage not null', () => {
+        const plainText = message.plainTextBuffer();
+        const decoded = SignalService.Content.toObject(SignalService.Content.decode(plainText));
+
+        expect(decoded.nullMessage).to.be.not.equal(null, 'decoded.dataMessage.nullMessage should not be null');
+    });
+
+    it('ttl of 5 minutes', () => {
+        expect(message.ttl()).to.equal(5 * 60 * 1000);
+    });
+});