Investigate and fix encryption in sample app

[ikurek]

When investigating #207 with @ikbalkaya we've noticed that encrypted messages don't work in the sample application. When an encrypted message is sent it appears decrypted in the sample app:

!https://user-images.githubusercontent.com/9201746/154529765-4736cbef-1f08-4c16-9be6-6285f59e1952.png!

I've followed the hint in the sample application that states:

You can confirm messages are encrypted by visiting your application dashboard on ably.com > "Dev console" tab > "Channels", and attach to the "encrypted-test-channel" channel.

But on the web panel, the messages also don't appear encrypted:

!https://user-images.githubusercontent.com/9201746/154530988-71da8018-568f-4e3b-b318-3b62a40af2a9.png!

There's probably some sort of bug in the sample application causing messages that should be encrypted to be sent in plaintext